updates and bug fixes ahead of next version of VACs

[moustakas]

This PR includes a mix of changes and updates under the umbrella of prepping for the next version of VACs. (I was hoping to include some additional documentation and unit tests but I will do that in a separate PR.)

Key updates include:

• Bash and slurm scripts (with hard-coded paths) used during production have been pulled out of the repo, following @sbailey's recommendations in get target cutouts using image coadds on-disk #108.
• NERSC- and MPI-friendly changes have been incorporated, such as not using subprocess and using multiprocessing.set_start_method('spawn').
• fastspecfit-qa has been renamed fastqa and the QA code has been ported to a new module, fastspecfit.qa.
• Updates to the default K-corrections and rest-frame photometry have been made, following the recommendations in review the set rest-frame bandpasses and band shifts #141.
• FLYA_1215_CONT, LOGL_1450, LOGL_1700, and LOGL_3000 have been added to the data model (see Wish list of outputs for a QSO VAC #112).
• {LINENAME}_BOXFLUX_IVAR and {LINENAME}_FLUX_IVAR have been updated / corrected, following the analysis in Equivalent width uncertainties are significantly lower in v2 #127 (comment).
• The data model documentation and web-app pages have been updated.