refactor: file tree with systems defninitions refactored to limit ODR issues

Relates to #32 as with this commit we created something like modules for derived quantities

src/include/units/generic/angle.h

@@ -0,0 +1,47 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2018 Mateusz Pusz
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#pragma once
+
+#include <units/physical/si/prefixes.h>
+#include <units/quantity.h>
+
+namespace units {
+
+struct radian : named_unit<radian, "rad", physical::si::prefix> {};
+
+template<Unit U = radian>
+struct dim_angle : base_dimension<"A", U> {};
+
+template<Unit U, ScalableNumber Rep = double>
+using angle = quantity<dim_angle<>, U, Rep>;
+
+inline namespace literals {
+
+// rad
+constexpr auto operator"" _q_rad(unsigned long long l) { return angle<radian, std::int64_t>(l); }
+constexpr auto operator"" _q_rad(long double l) { return angle<radian, long double>(l); }
+
+
+}  // namespace literals
+
+}  // namespace units