Release v2.3.0 about units

[nholthaus]

RC2 changes:

• Improved compile times
  • MSVC: 512%
  • GCC: 165%
• #63 Fixed constexpr relational operators bug

Retained VS2013 support.

[pvaibhav]

Something I did to help compile-time was to wrap all non-SI units in a disabled-by-default preprocessor #define. 99% of the time I don't need to deal with stones and furlongs and this improved my compilation time significantly on GCC-ARM.

Another suggestion I have is to split each category of units into its own header, e.g. one for voltages, current, impedance etc.; one for gravity-related etc.

I've not yet tried 2.3 RC2 (or 2.2 for that matter), but will do soon to see how much faster it can be now.

[nholthaus]

I have an investigation open to do exactly what you suggest in version 3. I'm trying to do a little work before hand to see why the compile times scale with unit definitions (e.g. if there are more problems like the operator<< pollution), but assuming it's unavoidable I'll probably just end up splitting the definitions.

You may not see a lot of compile-time improvement in v2.3.0 if you are already disabling iostream, because most of that speed-up came from un-polluting the ostream<< resolution in the global namespace.