Add a RSR convert script for FY-3G MERSI-RM sensor

[yukaribbba]

Source: https://img.nsmc.org.cn/PORTAL/NSMC/DATASERVICE/SRF/FY3G/FY-3G_MERSI-RM_SRF.zip

Result: rsr_mersirm_FY-3G.zip

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.00%. Comparing base (677af34) to head (fc249dc). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #222 +/- ## ======================================= Coverage 90.00% 90.00% ======================================= Files 23 23 Lines 2571 2571 ======================================= Hits 2314 2314 Misses 257 257 ``` | [Flag](https://app.codecov.io/gh/pytroll/pyspectral/pull/222/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/pytroll/pyspectral/pull/222/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `90.00% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll#carryforward-flags-in-the-pull-request-comment) to find out more.

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