Add altitude, landcover, and landseamask to mersi_ll_l1b reader

[chorng]

Add Altitude, LandCover, and LandSeaMask bands to mersi_ll_l1b reader for FY3E

• Closes Altitude, LandCover, and LandSeaMask are missing in the mersi_ll_l1b reader for FY3E L1B #2952
• Tests added
• Fully documented
• Add your name to AUTHORS.md if not there already

[djhoese]

Looks good to me. @simonrp84 thoughts?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.08%. Comparing base (f36a096) to head (392267f).
Report is 279 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2953   +/-   ##
=======================================
  Coverage   96.08%   96.08%           
=======================================
  Files         377      377           
  Lines       55125    55125           
=======================================
  Hits        52967    52967           
  Misses       2158     2158           

Flag	Coverage Δ
behaviourtests	3.94% <ø> (ø)
unittests	96.18% <ø> (ø)

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[coveralls]

Pull Request Test Coverage Report for Build 11662290144

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.191%

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Totals
Change from base Build 11634318306:	0.0%
Covered Lines:	53211
Relevant Lines:	55318

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💛 - Coveralls

[chorng]

Fixed conflict

[djhoese]

This looks good enough to me. If @simonrp84 has comments after the fact then we can adjust things then in a new PR. Merging now. Thanks @chorng!