Add handling of conditional includes

[mrbean-bremen]

• needed for validation of SR Document Content
• closes Include conditions not handled #39

[codecov-commenter]

Codecov Report

Merging #43 (760dc62) into main (c6ef9f4) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
+ Coverage   88.04%   88.14%   +0.09%     
==========================================
  Files          14       14              
  Lines        1489     1501      +12     
==========================================
+ Hits         1311     1323      +12     
  Misses        178      178              

Files Changed	Coverage Δ
dicom_validator/spec_reader/part3_reader.py	96.77% <100.00%> (+0.12%)	⬆️
dicom_validator/validator/iod_validator.py	93.11% <100.00%> (+0.07%)	⬆️

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