Embedded Version Number (2.0.0-alpha)

[ptheywood]

Embeds the FLAME GPU version number in the c++ headers and python module.

• Define the Major, Minor, Patch and Pre-release version components in CMake
  • Initially 2.0.0-alpha, to be followed by 2.0.0-alpha.1, 2.0.0-beta[.N], 2.0.0-pr.N and then 2.0.0
  • For now this will be defined in CMake rather than the header file itself, which is currently dynamically generaetd
• Embed the version number as a macro in a header file
  • FLAMEGPU_VERSION - a 7+ digit integer endcoding major, minor and patch, if multiple API versions are intended to be supported
• static constexpr for MAJOR, MINOR, PATCH, PRERELEASE and BUILDMETADATA in the flamegpu namespace
  • BUILDMETADATA is the old git commit hash.
• Embed the namespaced members into the python wheel
• Factor in the release version number in RTC comparisons, including the metadata.
• Document how the version numbers work (SemVer, -alpha[.N] for very early pre-releases, -rc[.x] for almost stable releases candidates
• Rename flamegpu2 CMake target to flamegpu, as it will/may eventually become 3.x.y

This will close #591 and close #163.

In the future, the location of the version definiation, and checking of the include directory metadata may change (see #600)