Move time step checker to BaseCase
The empty shell of check_timestep in BaseCase has been filled with what was essentially the same thing in each case file. Again, this is just more case file house cleaning. A few ancillary functions have also been written to help. The most important might be get_dt_max() which returns dt_max. dt_max can either be specified in spins.conf or defined in the case file based on the buoyancy frequency. Currently, if dt_max > 0 and in spins.conf, then that value is used. Otherwise, 0.5/sqrt(N2_max) is used. The re-definition of check_timestep in each case file can now be removed so long as the associated ancillary functions and parameters are included.
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