This commit pertains to a series of modifications that clean up
  to currently existing isobaric diagnostics interpolation. The
  existing diagnostics comprise a number of variables at individual
  pressure levels (e.g., temperature_200hPa) and thus require a significant
  amount of hard-coding.

  This modified version reduces (but does not eliminate) the need for
  hard-coding by prescribing a list of isobaric levels to which the
  variables are commonly interpolated, leading to variables of the form
  e.g., temperature_isobaric. These variables are also connected to a
  diagnostic package and namelist config option (config_isobaric = .true.)
  to make it easier for users to toggle on their calculation at runtime.

  The specific changes include:

  - Adding an isobaric package to Registry.xml

  - Adding the config_isobaric namelist option to Registry_diagnostics.xml

  - Connecting the package variables to the namelist config option in a new script,
    mpas_atm_diagnostics_packages.F, which is called in mpas_atm_core_interface.F

  - Updating Registry_isobaric.xml to remove the highly specific variables and
    replace them with more generalizable variables, such as temperature_isobaric

  - Updating Registry.xml to change the default diagnostics stream variables

  - Updating mpas_isobaric_diagnostics.F with the interpolation of the new
    generalizable variables and removal of the old variables

  - Adding halo groups 'isobaric:pressure_p' and 'isobaric:vorticity' to
    mpas_atm_halos.F and removing the previous halo field exchanges from
    mpas_isobaric_diagnostics.F

  - Updating mpas_atm_diagnostics_manager.F to include the halo exchange
    and config input arguments to the isobaric subroutine calls

  - Updating mpas_atm_core.F to include the halo exchange input arguments
    to mpas_atm_diag_compute()

Added package associations to `mslp` and `meanT_500_300`

Further cleaned up the script by defining subroutines for each interpolation procedure (cell centers on mass levels, vertices on mass levels, and cell centers on w levels) and for the temperature and dewpoint calculations.

Fixed division by 100. for garbage cell. Pressure is already in hPa.