Start of work on new systematic factory design

[marcpaterno]

Description

Redesign of factories for creation of systematics. This makes the factories more flexible in that the nature of the measurement to which the systematic will be applied partially determines what kind of systematic will be created.

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

The following checklist will make sure that you are following the code style and
guidelines of the project as described in the
contributing page.

• I have run bash pre-commit-check and fixed any issues
• I have added tests that prove my fix is effective or that my feature works
• I have made corresponding changes to the documentation
• I have 100% test coverage for my changes (please check this after the CI system has verified the coverage)

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.2%. Comparing base (4cd7052) to head (072b466).

Files with missing lines	Patch %	Lines
firecrown/updatable.py	66.6%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #471     +/-   ##
========================================
- Coverage    99.9%   97.2%   -2.7%     
========================================
  Files          54      54             
  Lines        4163    4162      -1     
========================================
- Hits         4160    4047    -113     
- Misses          3     115    +112     

Files with missing lines	Coverage Δ
firecrown/ccl_factory.py	100.0% <100.0%> (ø)
firecrown/likelihood/number_counts.py	92.9% <100.0%> (-6.6%)	⬇️
firecrown/likelihood/source.py	100.0% <100.0%> (ø)
firecrown/parameters.py	83.8% <100.0%> (-16.2%)	⬇️
firecrown/updatable.py	91.4% <66.6%> (-8.6%)	⬇️

... and 12 files with indirect coverage changes