Fix Global Multiplier when set to 0 (resolves #23)

[jvivian]

• Consolidate codebase's run-model function into core library
• Fix minor parameter menu bug in Factor Analysis
• Add test for global-multiplier-0

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (e338cc0) 87.5% compared to head (b5cc8eb) 75.2%.

Files	Patch %	Lines
covid19_drdfm/dfm.py	82.1%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main     #24      +/-   ##
========================================
- Coverage   87.5%   75.2%   -12.3%     
========================================
  Files          5       5              
  Lines        168     182      +14     
  Branches      30      35       +5     
========================================
- Hits         147     137      -10     
- Misses        18      44      +26     
+ Partials       3       1       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.