desisim 0.21.0

• Major refactor of newexp to add connection to upstream mocks, surveysims,
  and fiber assignment (PR #250).
• Support latest (>DR4) data model in the templates metadata table and also
  scale simulated templates by 1e17 erg/s/cm2/Angstrom (PR #252).
• Add desi_qa_s2n script (PR #254)
• Refactor desi_qa_zfind script (PR #254)
• Refactor redshift QA for new data model (PR #254)
• Refactor shared QA methods to desisim.spec_qa.utils (PR #254)
• New plots for S/N of spectra for various objects (ELG, LRG, QSO) (PR #254)
• Add BGS, MWS to z_find QA
• Miscellaneous polishing in QA (velocity, clip before RMS, extend [OII] flux, S/N per Ang)
• Bug fix: correctly select both "bright" and "faint" BGS templates by default
  (PR #257).
• Updates for newexp/fastframe wrappers for end-to-end sims (PR #258).