Update physics and astronomy field keywords to reduce false positives

[dfm]

I've been finding that we've been getting a fair number of incorrect track assignments for general physics papers in the AASS track. This is because the labels "General Physics and Astronomy" and "Physics and Astronomy (misc)" were both being sent to AASS. This PR splits those field labels to assign "Physics" to the PE track and "Astronomy" to the AASS track.

cc @kyleniemeyer

[arfon]

Looks good @dfm! @xuanxu – what is the process for re-importing the subjects? Can we just re-run the rake task?

[xuanxu]

@arfon to re-import just run the tracks:import:subjects task. Before that you can run tracks:import:dry_run to make sure it'd work and get some info on what will be imported

[arfon]

OK, those should be imported now @dfm

[dfm]

Thanks @arfon and @xuanxu!!