File handling for gamutrf-offline

[lk-iqt]

Bug: cannot process recordings in current directory.
Defaults to "." if no out_dir found.

Bug: data_filename does not contain file extension. Per sigmf format - datafiles must end with .sigmf-data

https://github.com/sigmf/SigMF/blob/sigmf-v1.x/sigmf-spec.md#sigmf-file-types
Rules for SigMF Dataset files:
The Dataset file MUST have a .sigmf-data filename extension.

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (0b35ebf) 76.44% compared to head (a1b70ad) 76.32%.
Report is 5 commits behind head on main.

Files	Patch %	Lines
gamutrf/sample_reader.py	0.00%	4 Missing ⚠️
gamutrf/offline.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1035      +/-   ##
==========================================
- Coverage   76.44%   76.32%   -0.13%     
==========================================
  Files          43       43              
  Lines        3630     3636       +6     
==========================================
  Hits         2775     2775              
- Misses        855      861       +6     

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