Misc fixes for nholthaus#122 (nholthaus#140)

* s/RTTI/compile-time

* Change friendship to private label

Indicates that it's an implementation detail,
although friendship isn't affected by accessibility.

* Make the constants inline

And remove the implied static specifier.

* Remove implied inline specifiers

variadic dimensions (nholthaus#111)

* c++17-ization
* removed vs2013
* fixed gtest compilation
* usedconstexpr if to simplify the conversion function and removed the SFINAE
* renamed `unit_t` to `unit`
* more c++17. Renamed unit_tag to unit_conversion
* clarified template args
* improved error messages
* constexpr sqrt/pow

v2.3.1 documentation

v2.3.x development

* added compound assignment operators: +=, -=, *=, /=. Closes nholthaus#44.

* print undefined unit types in SI base units. Closes nholthaus#38. Update constants to NIST 2014 CODATA recommendations.

* Reduce metaprogramming boilerplate

* workaround for vs2013 internal compiler error

* added data and data transfer units. Closes nholthaus#46.

* update doxyfile.in to v2.2.0

* add constexpr std::array test

* add julian and gregorian year definitions. Closes nholthaus#29.

* added `value()` and `value_type` trait. Closes nholthaus#56.

* fixes constexpr relational operator bug. Closes nholthaus#63.

* improve compile time.

* removed some debug code.

* measure compile time

* Friend the linear scale units. Reduces compile time.

* add inline to pow() and abs() for being able to use units.h in different compilation units

* Revert "Friend the linear scale units. Reduces compile time."

This reverts commit e26fcd7.

* Revert "add inline to pow() and abs() for being able to use units.h in different compilation units"

This reverts commit d42020e.

* add inline to pow() and abs() for being able to use units.h in different compilation units

* Test with Visual Studio 2017

* Another attempt at using Visual Studio 2017 in appveyor

* Build with VS2017

* add MSVC2017 badge

* started min/max

* fixes nholthaus#65, fixes nholthaus#71, fixes nholthaus#76, fixes nholthaus#79, fixes nholthaus#80.  Adds documentation.

* fix for vs2013, gcc

* updated docs

* fix for vs2013

* remove gcc warnings, add prefined unit defines

* min/max shouldn't be constexpr

* fix cmake

* more cmake fixes

* numeric_limits namespace

* fixes nholthaus#77.

* fixes nholthaus#75 & documentation

* fixed merge issues

* for Morwenn

removed some debug code.

added `value()` and `value_type` trait. Closes nholthaus#56.

Merge pull request nholthaus#53 from nholthaus/v2.2.x

merge v2.2.x development into master

cmake improvements (nholthaus#24)

- adds an `INTERFACE` cmake library for easier consumption as a subproject
- Removes GNU dependencies
- Updates google test to 1.8.0 and substantially simplifies the google test build
- adds caching to our CI system to improve build speed
- Add options to avoid building the unit tests/documentation (this will be helpful for subdirectories)

Merge pull request nholthaus#22 from nholthaus/inline_dB

Fixes nholthaus#21

Merge pull request nholthaus#19 from nholthaus/clang

Add clang support