feat: bug: fug: mom6 perturbation model specific perturbation

[hkershaw-brown]

Perturbing MOM6 to get some ensembles to play with.
Currently (lazily) using the default perturbation routine, which perturbs everything in the state including land.

This gives some fuzz on the land:

c.TL319_t232.001

nwa12_test.3_0_a03a

Compare to non-fuzz before perturbation

who cares?
You'll inflate the land because mean /= 0 (adds more fuzz), get_close makes 'dry' state far away so the assimilation update doesn't change the land, but still I think dangerous to fuzz.
See #359 for missing_in_state comment that I have not rewritten yet. #359 (comment)

[hkershaw-brown]

note: land is land+ocean floor

todo @HKershaw get_state_meta_data / get_close_state should be checking ocean floor?

This ocean floor check wrong/not needed?

DART/models/MOM6/model_mod.f90

Lines 547 to 549 in a85e162

	lon_lat_vert = get_location(locs(close_ind(ii))) ! assuming VERTISHEIGHT
	call get_model_variable_indices(loc_indx(ii), i, j, k)
	if ( below_sea_floor(i,j,lon_lat_vert(3)) ) dist = 1.0e9_r8

[hkershaw-brown]

I'm not sure what the MOM6 grid looks like:

2D wet/dry sufficient (squishy bottom grid follows ocean basin) vs. 3D wet/dry (grid extends into ocean basin)

[hkershaw-brown]

https://github.com/NCAR/DART/tree/pert_plus_missing-grid has pert_model_copies & missing grid from #727 so it can run