feat: add caching

[tlambert03]

this is laying the groundwork for a general cache mechanism. for caching ground truths, psfs, etc...

Caching is on by default, but can be disabled per sim with Settings(cache=False) or the environment variable MICROSIM_CACHE=0

closes #23
relates to #25

[codecov]

Codecov Report

Attention: Patch coverage is 92.00000% with 10 lines in your changes missing coverage. Please review.

Project coverage is 80.45%. Comparing base (31e68bf) to head (267e008).

Files	Patch %	Lines
src/microsim/util.py	50.00%	7 Missing ⚠️
src/microsim/schema/settings.py	78.57%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #41      +/-   ##
==========================================
- Coverage   81.39%   80.45%   -0.95%     
==========================================
  Files          42       43       +1     
  Lines        2005     2118     +113     
==========================================
+ Hits         1632     1704      +72     
- Misses        373      414      +41     

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[tlambert03]

important before merge: make this optional. environment variable and/or other things