implement nested run sequences

[ghost]

implement nested run sequences and arbitrary FORTRAN functions, as in this config:
https://github.com/NOAA-EMC/DATM-MOM6-CICE5/blob/ca3d21bd89f967e4c25953eaa3349829c739b528/parm/nems.configure.med_atm_ocn_ice.IN#L51-L70

# Run Sequence #
runSeq::
  @@[coupling_interval_slow_sec]
    MED MedPhase_prep_ocn
    MED -> OCN :remapMethod=redist
    OCN
    @@[coupling_interval_fast_sec]
      MED MedPhase_prep_ice
      MED -> ICE :remapMethod=redist
      ATM
      ICE
      ATM -> MED :remapMethod=redist
      ICE -> MED :remapMethod=redist
      MED MedPhase_atm_ocn_flux
      MED MedPhase_accum_fast
    @
    OCN -> MED :remapMethod=redist
    MED MedPhase_write_restart
  @
::

[ghost]

need to figure out how many sequences can be nested per-sequence, and how many layers deep the nesting can go

[ghost]

it seems that nested sequences run with even multiple time intervals of their containing sequence (i.e. a 30 minute sequence will run twice inside a 1 hour sequence)

[ghost]

pull request #15 implemented custom functions as members of a defined mediator object