refactor: References are now guarded UNITS_REFERENCES with (ON by default) + examples duplicated to subdirectories

Now References can be disabled to meassure a compile time impact. Also the same examples are now provided in two subdirectories to be able to easily compare the pros and cons of every quantity construction technique.

example/capacitor_time_curve.cpp

@@ -1,63 +0,0 @@
-/*
- Copyright (c) 2003-2020 Andy Little.
-
- This program is free software: you can redistribute it and/or modify
- it under the terms of the GNU General Public License as published by
- the Free Software Foundation, either version 3 of the License or
- (at your option) any later version.
-
- This program is distributed in the hope that it will be useful,
- but WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
- GNU General Public License for more details.
-
- You should have received a copy of the GNU General Public License
- along with this program. If not, see http://www.gnu.org/licenses./
-*/
-
-/*
-    capacitor discharge curve using compile_time
-    physical_quantities
-*/
-
-#include <units/generic/dimensionless.h>
-#include <units/isq/dimensions/electric_current.h>
-#include <units/isq/si/capacitance.h>
-#include <units/isq/si/resistance.h>
-#include <units/isq/si/time.h>
-#include <units/isq/si/voltage.h>
-#include <units/math.h> // IWYU pragma: keep
-#include <units/quantity_io.h>
-#include <iostream>
-
-int main()
-{
-  using namespace units::isq;
-  using namespace units::isq::si::references;
-
-  std::cout << "mp-units capacitor time curve example...\n";
-  std::cout.setf(std::ios_base::fixed, std::ios_base::floatfield);
-  std::cout.precision(3);
-
-  constexpr auto c = 0.47 * uF;
-  constexpr auto v0 = 5.0 * V;
-  constexpr auto r = 4.7 * kR;
-
-  for (auto tt = 0 * ms; tt <= 50 * ms; ++tt) {
-    const Voltage auto vt = v0 * units::exp(-tt / (r * c));
-
-    std::cout << "at " << tt << " voltage is ";
-
-    if (vt >= 1 * V)
-      std::cout << vt;
-    else if (vt >= 1 * mV)
-      std::cout << quantity_cast<si::millivolt>(vt);
-    else if (vt >= 1 * uV)
-      std::cout << quantity_cast<si::microvolt>(vt);
-    else if (vt >= 1 * nV)
-      std::cout << quantity_cast<si::nanovolt>(vt);
-    else
-      std::cout << quantity_cast<si::picovolt>(vt);
-    std::cout << "\n";
-  }
-}