[wpiutil] Split units.h into separate headers for each unit (#2551)

Closes #2508.

Co-authored-by: Prateek Machiraju <prateek.machiraju@gmail.com>

wpiutil/src/main/native/include/units/concentration.h

@@ -0,0 +1,59 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2020 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+// Copyright (c) 2016 Nic Holthaus
+//
+// The MIT License (MIT)
+//
+// 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/base.h"
+
+namespace units {
+/**
+ * @namespace units::concentration
+ * @brief namespace for unit types and containers representing concentration
+ *        values
+ * @details The SI unit for concentration is `parts_per_million`, and the
+ *          corresponding `base_unit` category is `scalar_unit`.
+ * @anchor concentrationContainers
+ * @sa See unit_t for more information on unit type containers.
+ */
+#if !defined(DISABLE_PREDEFINED_UNITS) || \
+    defined(ENABLE_PREDEFINED_CONCENTRATION_UNITS)
+UNIT_ADD(concentration, ppm, parts_per_million, ppm,
+         unit<std::ratio<1, 1000000>, units::category::scalar_unit>)
+UNIT_ADD(concentration, ppb, parts_per_billion, ppb,
+         unit<std::ratio<1, 1000>, parts_per_million>)
+UNIT_ADD(concentration, ppt, parts_per_trillion, ppt,
+         unit<std::ratio<1, 1000>, parts_per_billion>)
+UNIT_ADD(concentration, percent, percent, pct,
+         unit<std::ratio<1, 100>, units::category::scalar_unit>)
+
+UNIT_ADD_CATEGORY_TRAIT(concentration)
+#endif
+
+using namespace concentration;
+}  // namespace units