Template ghgrmhd on neutrino transport stub

[pajkosmi]

Proposed changes

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• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
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Further comments

[pajkosmi]

@nilsdeppe this can be reviewed at your leisure. I am just waiting on the comparison runs on mbot to finish. I should know the results by tomorrow.

[pajkosmi]

The TOV central density evolutions for the early portion of the evolution match between the GhValenciaDivClean and GhValenciaDivClean<NeutrinoStubSystem> cases.

[nilsdeppe]

LGTM! Does this need to be squashed or do the individual commits compile?

[pajkosmi]

Rebased and squashed