Make temperature and pressure coupling enums enum classes

Make temperature and pressure coupling enums enum classes

This makes the plain enums denoting the temperature and pressure
coupling enums enum classes. Most of this change is simple renaming,
changing etcXXX into TemperatureCoupling::XXX, and epcXXX into
PressureCoupling::XXX.

As these are the first of many enums in md_enums.h being refactored
into enum classes, this also introduces some reusable functionality,
including a templated function mapping enums to a string representing
the functionality, and helper functions used when reading inputrec
from file or comparing inputrec instances.

This keeps explicit static_casts from / to int in two places: When
serializing the inputrec, and when transferring the values over
multiple simulations.