[newchem-cpp] cppify initialize_UVbackground_data and initialize_cloudy_data

[mabruzzo]

This PR converts initialize_UVbackground_data.c to initialize_UVbackground_data.cpp and initialize_cloudy_data.c to initialize_cloudy_data.cpp.

The tweaks to the source code are EXTREMELY straightforward and formulaic.

I made 3 significant changes:

1. I make brief use of std::vector inside of initialize_UVbackground_data. This change was made to replace the use of a gcc compiler extension (and std::vector was the easiest way to replicate the prior behavior)
2. I factored out the logic for cleaning up the allocations in my_rates->UVbackground_table. This is now handled by a function called grackle::impl::free_UVBtable (this was done for better modularity and better consistency with other datatypes)
3. I moved the call to initialize_empty_UVBtable_struct so that it occurs at the start of initialize_UVbackground_data (for reference, we previously called initialize_empty_UVBtable_struct immediately before we called initialize_UVbackground_data -- this won't change anything about how the codebase actually works)