Third party examples : add box example

Add si::density quantity header .
Add si::resistance quantity header.
Update si::capacitance header with mF, uF, nF, pF.
Update si::voltage header with mV, uV, nV,pV
Third party example : add capacitor time curve example
Add incoherent length units, TODO  move them out from si header.
Third party examples : add clcpp_response showing effectivenes of typed units for physical quantity library
Third party examples : add conversion factor example
Add third party examples to cmake

Third party examples : box example : Add air_density constant for clarity remove explicit this-> and tidy up.
Third party examples : in clcpp response example, change base unit from km to m for single type or all units example.
Third party examples : conversion_factor , add inline constexpr to units_str function.
Third party examples : box_example, change quantity::unit syntax to quantity::unit<> to allow generic(default double) value_type.
examples : remove examples from third party to main examples directory. Update cmake.
physical/si/resistance.hpp : remove underscores from kiloohm etc, UDL collision with 'R' so prefix with underscore

example/box_example.cpp

@@ -0,0 +1,125 @@
+
+
+#include <units/physical/si/acceleration.h>
+#include <units/physical/si/length.h>
+#include <units/physical/si/volume.h>
+#include <units/physical/si/time.h>
+#include <units/physical/si/force.h>
+#include <units/physical/si/mass.h>
+#include <units/physical/si/density.h>
+#include <cassert>
+
+namespace{
+
+   namespace length{
+
+      template <typename Rep = double>
+      using m = units::si::length<units::si::metre,Rep>;
+
+      template <typename Rep = double>
+      using mm = units::si::length<units::si::millimetre,Rep>;
+   }
+   
+   namespace acceleration{
+
+      template <typename Rep = double>
+      using m_per_s2 = units::si::acceleration<units::si::metre_per_second_sq,Rep>;
+
+      template <typename Rep = double>
+      constexpr m_per_s2<> g{static_cast<Rep>(9.80665)};
+   }
+
+   namespace force{
+
+      template <typename Rep = double>
+      using N = units::si::force<units::si::newton,Rep>;
+   }
+
+   namespace mass {
+
+      template <typename Rep = double>
+      using kg = units::si::mass<units::si::kilogram,Rep>;
+   }
+
+   namespace density {
+
+      template <typename Rep = double>
+      using kg_per_m3 = units::si::density<units::si::kilogram_per_metre_cub,Rep>;
+   }
+
+   namespace volume {
+
+      template <typename Rep = double>
+      using m3 = units::si::volume<units::si::cubic_metre,Rep>;
+   }
+}
+
+struct Box{
+
+    Box(length::m<> const& l,
+        length::m<> const& w,
+        length::m<> const& h
+    ): length{l},width{w},height{h}{}
+
+    force::N<> filled_weight()const
+    {
+        volume::m3<> const volume
+           = length * width * height;
+        mass::kg<> const mass = contents.density * volume;
+        return mass * acceleration::g<>;
+    }
+
+    length::m<> fill_level(mass::kg<> const & measured_mass)const
+    {
+       return height
+          * (measured_mass * acceleration::g<>) / filled_weight();
+    }
+
+    volume::m3<> spare_capacity(mass::kg<> const & measured_mass)const
+    {
+       return (height - fill_level(measured_mass)) * width * length;
+    }
+
+    struct contents{
+        contents():density{air_density}{}
+        density::kg_per_m3<> density;
+    }contents;
+
+    void set_contents_density(density::kg_per_m3<> const & density_in)
+    {
+        assert( density_in > air_density );
+        contents.density = density_in;
+    }
+
+    static constexpr density::kg_per_m3<> air_density{1.225};
+
+    length::m<> length;
+    length::m<> width;
+    length::m<> height;
+};
+
+#include <iostream>
+
+using namespace units::si::literals;
+int main()
+{
+   auto box = Box{1000.0mm, 500.0mm, 200.0mm};
+   box.set_contents_density(1000.0kg_per_m3);
+
+   auto fill_time = 200.0s;      // time since starting fill
+   auto measured_mass = 20.0kg;  // measured mass at fill_time
+
+   std::cout << "mpusz/units box example...\n";
+   std::cout << "fill height at " << fill_time << " = "
+   << box.fill_level(measured_mass) << "( " << (box.fill_level(measured_mass)/ box.height)*100 << "% full)\n";
+   std::cout << "spare_capacity at " << fill_time << " = "
+   << box.spare_capacity(measured_mass) <<'\n';
+   std::cout << "input flow rate after " << fill_time
+   << " = " << measured_mass / fill_time <<'\n';
+   std::cout << "float rise rate = "
+   << box.fill_level(measured_mass) / fill_time <<'\n';
+   auto fill_time_left
+   = (box.height / box.fill_level(measured_mass) - 1) * fill_time ;
+   std::cout << "box full E.T.A. at current flow rate = " << fill_time_left <<'\n';
+
+}