Enforce naming convention for SurfEmiss

[smnewman]

The IODA naming conventions define the group SurfEmiss for surface emissivity:
https://github.com/JCSDA-internal/ioda/blob/develop/share/ioda/yaml/validation/ObsSpace.yaml#L112.
However, in opsinputs VarField_mwemiss is mapped to Emiss/emissivity.
https://github.com/MetOffice/opsinputs/blob/develop/src/opsinputs/opsinputs_varobswriter_mod.F90#L861

Accompanied by the change Emiss/emissivity -> SurfEmiss/emissivity for yamls in sith, source code in opsinputs should reflect the new naming convention.

See https://github.com/MetOffice/sith/issues/163.

[smnewman]

@mikecooke77 I had forgotten about this issue. As of now, it doesn't seem too important to me. We could allow the old-style name to persist in yamls for VarObs Writer. What do you think?

[mikecooke77]

We could do. I think at some point it would be nice to deal with but if its get forgotten in the sands of time then so be it. I think worth leaving the issue open.

[ctgh]

@smnewman / @mikecooke77 , do you still want to keep this issue open? No problem if so, just checking.