Add new platforms and clarify help message for check_platform

[simonrp84]

This PR adds several satellites to the platforms.txt file. The sentinels (S1B, S2A/B) are added along with GOES-17, -18 and -19. I also added some missing Chinese satellites (FY-3E/F/G).

In addition, I updated the help message for the check_platform.py script to state that the satellite name must be given in upper case, as this confused me when I first ran the script.

Lastly, I updated the PR template to replace master with main.

• Closes SENTINEL-2 TLE files #160
• Tests passed
• Passes flake8 pyorbital
• Fully documented

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.12%. Comparing base (14db2a9) to head (03580df).
Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #161   +/-   ##
=======================================
  Coverage   88.12%   88.12%           
=======================================
  Files          14       14           
  Lines        2265     2265           
=======================================
  Hits         1996     1996           
  Misses        269      269           

Flag	Coverage Δ
unittests	88.12% <ø> (ø)

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[coveralls]

coverage: 88.06%. remained the same
when pulling 03580df on simonrp84:AddSentinel
into 18abb61 on pytroll:main.

[adybbroe]

LGTM, Thanks! Not often I use the check_platforms.py script, didn't realize/remember it required uppercase!