Add new step slit_cube_build for building NIRSpec MSA cubes

[jemorrison]

Helps to resolve JP-4388

This PR is a draft - submitted to get early feed back. There are some print statements that have been left to aid in testing that need to be removed and few other issue that need to be cleaned up.

This PR addresses add a new step, slit_cube_build, to create NIRSpec MSA cubes.
By default MSA cube build will weight by overlap volume and build cubes in a "drizzle fashion". There is a desire to
add read noise weighting. There is a module msa_cube_overlap.py that is just temporary. This code performs drizzle cube building weighting by overlap area and read noise. The cube_build C routines need to be updated to allow weighting by read noise. For this PR I can remove the option to weight by read noise and run this module. Or we can update the C cube build code
to allow this.

In addition we should create a new output model 'msacube'. Currently we are using the ifucube model, but there are some new
parameters (slit_frac) that are not part of the ifucube model that we need to record and we are creating a new plane called
ETIME (which can be used to find the effective exposure time of very spaxel).
There is a PR out for cube_build changing how the DQ plane is formed. Once that is merged we need to add some of the hooks
in the code (IF DESIRED) to send saturated data to the C code.

Tasks

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