Merge pull request #353 from nholthaus/v3.x-pct-semantics

V3.x pct semantics

feat: NTTP support

feat: update to 3.1.2

- update gtest to 1.17.0 (to make cmake happy)

- update cmake support to 4.99
- better cmake exports
- add cpack for tar/deb/rpm

fix: Add jerk units to CMakeLists and readme (#341)

Signed-off-by: Andreas Rauschert <andreas.rb.rauschert@bmw.de>

api: normalize the unit acronyms for release

fix: better conversion for inHg

- Make `.value()` and `.to<double()` behave the same way for concentr…

…ation units

- Ensure consistency in concentration unit math functions
- Add substantially more testing of concentration units

Update units.h

fix: mil definition

fix: metric_tons renammed tonnes

feat: added simple unit test for std::complex unit types