Allow subclasses of UserObjects to be valid input even if they are defined after the object that references them

[jtv8]

Resolves #52

[codecov]

Codecov Report

Merging #53 (a3b387b) into master (f39237e) will decrease coverage by 0.02%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
- Coverage   91.59%   91.56%   -0.03%     
==========================================
  Files          30       30              
  Lines         547      557      +10     
==========================================
+ Hits          501      510       +9     
- Misses         46       47       +1     

Flag	Coverage Δ
unittests	91.56% <93.33%> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
wysdom/object_schema/SchemaAnyOf.py	90.32% <80.00%> (-2.54%)	⬇️
wysdom/__version__.py	100.00% <100.00%> (ø)
wysdom/user_objects/UserObject.py	98.03% <100.00%> (+0.31%)	⬆️

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