Add a read-only FSFile.fs property

[pnuu]

This PR exposes the private FSFile._fs via a read-only property which would be nicer than accessing the private attribute. I'll need this for using fsspec caching I'm adding to Trollflow2 in pytroll/trollflow2#205

• 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 95.94%. Comparing base (aff4f31) to head (52953f6).
Report is 332 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2832   +/-   ##
=======================================
  Coverage   95.94%   95.94%           
=======================================
  Files         366      366           
  Lines       53522    53531    +9     
=======================================
+ Hits        51350    51359    +9     
  Misses       2172     2172           

Flag	Coverage Δ
behaviourtests	4.04% <18.18%> (+<0.01%)	⬆️
unittests	96.03% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[coveralls]

Pull Request Test Coverage Report for Build 9584991230

Details

• 11 of 11 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 96.041%

---

Totals
Change from base Build 9520945640:	0.001%
Covered Lines:	51572
Relevant Lines:	53698

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9592336810

Details

• 11 of 11 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 96.042%

---

Totals
Change from base Build 9588363290:	0.001%
Covered Lines:	51588
Relevant Lines:	53714

---

💛 - Coveralls

[mraspaud]

LGTM