Add lenient loading and type system merging to load_cas_from_json

[timbmg]

• Add lenient loading parameter when loading CAS from JSON
• Add merge_typesystem parameter when loading CAS from JSON

[codecov[bot]]

Codecov Report

Merging #288 (7968342) into main (ed6b4f5) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #288   +/-   ##
=======================================
  Coverage   95.11%   95.11%           
=======================================
  Files           5        5           
  Lines        1966     1967    +1     
=======================================
+ Hits         1870     1871    +1     
  Misses         96       96           

Files	Coverage Δ
cassis/json.py	90.35% <100.00%> (+0.02%)	:arrow_up:

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[reckart]

@timbmg Thanks for the contribution 👍