Add APPEND mode to exodusIIcpp

[andrsd]

Implements support for opening existing ExodusII files in append mode to add new time steps to an existing file. This is useful for resuming simulations and continuing to write output.

Changes:

• Add FileAccess::APPEND enum value
• Add File::append() method that uses ex_open(..., EX_WRITE, ...)
• Update File constructor to handle APPEND mode
• Update init() to allow reading metadata in APPEND mode
• Expose append() in Python bindings
• Add comprehensive test that verifies appending time steps works correctly

Co-Authored-By: Claude Haiku 4.5 noreply@anthropic.com

[codecov]

Codecov Report

❌ Patch coverage is 98.07692% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.71%. Comparing base (55bdffc) to head (0da9904).
⚠️ Report is 8 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/file.cpp	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #86      +/-   ##
==========================================
- Coverage   99.84%   99.71%   -0.14%     
==========================================
  Files          13       13              
  Lines         654      705      +51     
==========================================
+ Hits          653      703      +50     
- Misses          1        2       +1     

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