Update 'oci_l2_bgc' to allow .nc4 and .nc filename extensions

[djhoese]

@kathys let me know that the files this reader is meant to read now come with a .nc extension. This PR fixes the reader to allow the older .nc4 extension and the newer .nc extension.

• Closes #xxxx
• Tests added
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.11%. Comparing base (da46066) to head (f7d5dba).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3055   +/-   ##
=======================================
  Coverage   96.11%   96.11%           
=======================================
  Files         383      383           
  Lines       55685    55685           
=======================================
  Hits        53523    53523           
  Misses       2162     2162           

Flag	Coverage Δ
behaviourtests	3.88% <ø> (ø)
unittests	96.21% <ø> (ø)

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[mraspaud]

LGTM