Unique Release Numbers

With a new release numbering scheme defined, there needs to be way to get the current version number into the program for output with the -v command line option (and eventually in the Help/About box once the GUI is implemented).  The version number should also be unique during development since not every commit will be tagged.  The goals are:

1. Use the current tag if at a tagged commit
2. Represent when not at a tagged commit during development
3. Use the current release number assigned for archive downloads
4. Allow for developmental (dash) and patch (period) numbering

The inclusion of the third goal will be clear shortly.  It turns out that the exact desired release string can be obtained using the git describe command, which returns the name of the most recent tag.  When beyond the most recent tag, the number of commits beyond the tag plus the short form of the current commit ID is appended.

This works as desired when the git command is available and the git repository information is present, but for the third goal, the downloaded archives have no git information.  Since the download archives are only available at tagged commits, the release number set in the CMakeLists.txt file can be used and will match the tag at that commit (assuming these variables were set to the same values as the tag).

For the last goal, there are two cases, git repository information present and not present.  When making tags during development, the tag name format will be releaseX.Y‑Z (note the dash).  The git describe command will pick this tag name (and append the rest if beyond that tagged commit).  When the git repository information is not present, the major, minor and patch release numbers set in the  CMakeLists.txt file will be used.  To handle developmental (dash) numbering, negative patch numbers will be used.