Clean out duplicated functions and variables

We have different functions and variables which have duplicates. We should clean that up to always use one version. Here some examples

• posydon.utils.gridutils.kepler3_a vs. posydon.utils.common_functions.orbital_separation_from_period
• posydon.utils.gridutils.T_merger_P vs. posydon.utils.common_functions.inspiral_timescale_from_orbital_period
• posydon.utils.gridutils.T_merger_a vs. posydon.utils.common_functions.inspiral_timescale_from_separation
• variables are often hard coded in functions instead of imported from where they are defined, this happens often in step_* vs. flow_chart