[newchem-cpp] convert solve_chemistry.{c->cpp} and calculate_*.{c->cpp}

[mabruzzo]

This should be reviewed after #411 is merged

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This is a lightweight PR where I converted the following c files to C++

• solve_chemistry.c -> solve_chemistry.cpp
• calculate_temperature.c -> calculate_temperature.cpp
• calculate_pressure.c -> calculate_pressure.cpp
• calculate_gamma.c -> calculate_gamma.cpp
• calculate_dust_temperature.c -> calculate_dust_temperature.cpp
• calculate_cooling_time.c -> calculate_cooling_time.cpp

I made some other light edits along the way. This should be quick and very easy to review.