Fix some issues in the condition parser

[mrbean-bremen]

• correctly parse the value index for some expressions
• make the parsing stricter to avoid false positives
• fix parsing of conditions for AT values with equality comparison (fixes BaseTag handling in IODValidator._tag_matches() #58)

[codecov-commenter]

Codecov Report

Merging #61 (732ec76) into main (64635f4) will increase coverage by 0.38%.
Report is 1 commits behind head on main.
The diff coverage is 98.43%.

❗ Current head 732ec76 differs from pull request most recent head 8523be9. Consider uploading reports for the commit 8523be9 to get more accurate results

@@            Coverage Diff             @@
##             main      #61      +/-   ##
==========================================
+ Coverage   88.93%   89.31%   +0.38%     
==========================================
  Files          14       14              
  Lines        1518     1554      +36     
==========================================
+ Hits         1350     1388      +38     
+ Misses        168      166       -2     

Files	Coverage Δ
dicom_validator/spec_reader/condition.py	95.23% <100.00%> (+0.03%)	⬆️
dicom_validator/spec_reader/condition_parser.py	98.68% <98.24%> (-0.21%)	⬇️

... and 1 file with indirect coverage changes

📣 Codecov offers a browser extension for seamless coverage viewing on GitHub. Try it in Chrome or Firefox today!