Fix regex, Doxygen, etc. invalid-escape warnings (v2)

[agbuckley]

From Python 3.6 onwards, strings containing invalid escape sequences lead to noisy warnings, since these were previously passed through safely but now can be misinterpreted as generic Unicode escapes rather than specific C/Python string sequences.

We're getting a lot of these from the regex-heavy harvester machinery, and from Doxygen backslash escapes (these could be @ signs) in generated headers. It's more than a build-cleanliness issue, since these warnings are scheduled to become syntax errors in an upcoming Python 3.x release.

I've attempted to fix them, using mainly raw r"" strings. Let's see if the CI flags any issues with the generated output.

[agbuckley]

Note that it's easier to view the diff with whitespace changes hidden, due to a lot of trailing spaces, particularly in the header generation. Click the settings cog in the GitHub "Files changed" tab.

[ChrisJChang]

Trying to build this locally fails. This seems to generate additional slashes that break the build. For example, in Models/src/particle_database.cpp:

#include \"gambit/Models/partmap.hpp\"
#include \"gambit/Models/particle_macros.hpp\"