Add new planetary equations of state and EoS functions

doc/RTD/source/Planetary/equations_of_state.rst

examples/Planetary/EoSTables/get_eos_tables.sh

examples/parameter_example.yml

src/equation_of_state/barotropic/equation_of_state.h

src/equation_of_state/ideal_gas/equation_of_state.h

src/equation_of_state/isothermal/equation_of_state.h

src/equation_of_state/planetary/equation_of_state.h

src/equation_of_state/planetary/hm80.h

src/equation_of_state/planetary/ideal_gas.h

src/equation_of_state/planetary/sesame.h

src/equation_of_state/planetary/tillotson.h

src/utilities.h