model_ID to diagnostics file

[AlexisRenchon]

It would be nice to have an optional argument to pass a model ID to diagnostics.

For example,

Attributes:
  units      => mol CO2 m^-2 s^-1
  short_name => gpp
  long_name  => Gross Primary Productivity, average within 1 Month
  start_date => 2008-03-01T00:00:00
  comments   => Gross photosynthesis (carbon assimilation) of the canopy.
  model_ID   => climaland_pmodel_976879

As this is optional, it wouldn't break anything.