Change FileStream to use absolute paths

[DWesl]

Description

Currently, accessing a grib file first by a relative path then by an absolute path causes cfgrib to reject the index, as would acessing the file by a relative path from a different working directory.

This change ensures the path stored in the index file is absolute, so the index file validator will find a match regardless of whether the file is accessed by a relative path or an absolute path.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 41.76%. Comparing base (44ecc62) to head (18639c7).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
cfgrib/messages.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #436   +/-   ##
=======================================
  Coverage   41.76%   41.76%           
=======================================
  Files          26       26           
  Lines        2227     2227           
  Branches      195      195           
=======================================
  Hits          930      930           
  Misses       1297     1297           

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