Add support for the MERSI-RM instrument on FY-3G

[simonrp84]

This PR adds support for the MERSI-RM instrument aboard Fengyun-3G.
Unfortunately it's quite a different instrument to MERSI-2 and MERSI-LL so the existing YAMLs couldn't be reused. Luckily, the actual python code requires minimal change.

• Tests added

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5320bca) 95.92% compared to head (ed751f8) 95.92%.
Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2720   +/-   ##
=======================================
  Coverage   95.92%   95.92%           
=======================================
  Files         375      375           
  Lines       53100    53167   +67     
=======================================
+ Hits        50936    51003   +67     
  Misses       2164     2164           

Flag	Coverage Δ
behaviourtests	4.13% <0.00%> (-0.01%)	⬇️
unittests	96.02% <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 7985771067

Details

• 0 of 74 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 96.009%

---

Totals
Change from base Build 7978811481:	0.005%
Covered Lines:	50875
Relevant Lines:	52990

---

💛 - Coveralls

[mraspaud]

LGTM, thanks for adding support for the RM.