Tabulated UVB/cooling needs refactoring

[brittonsmith]

There is so low hanging refactoring fruit in the tabulated UV background and cooling code that yield some minor optimizations, cut down on duplicate code, and standardize a few things. The list I can think of so far includes:

• refactor initialize_cloudy_data.c to use the read_dataset function implemented in initialize_UVbackground_data.c
• UVB rates should be converted to code units and logspace upon read in initialize_UVbackground_data.c like the cooling tables. This would prevent having to do this every time in update_UVbackground_rates.c
• update_UVbackground_rates.c could use a more efficient of calculating the interpolation index than the brute force for loop. For tables evenly spaced in log(1 + z), we can do this with algebra. Otherwise, bisection would also be an improvement.