Fix tropomi_l2 reader not using y and x dimension names

[djhoese]

sateesh (can't find github username) on slack pointed out that the tropomi_l2 reader can't be resampled. It fails with the error:

ValueError: cannot add coordinates with new dimensions to a DataArray

This was caused by the reader producing data that didn't include the y and x dimensions and instead was using the file's original dimensions scanline and ground_pixel. From what I can tell we don't mention this in the custom reader documentation. I'll make a separate issue or PR for that.

• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy

[coveralls]

Coverage increased (+0.005%) to 87.28% when pulling c15b72a on djhoese:bugfix-tropomi-dim-names into c80a510 on pytroll:master.

[codecov]

Codecov Report

Merging #1031 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1031      +/-   ##
==========================================
+ Coverage   87.27%   87.28%   +<.01%     
==========================================
  Files         188      188              
  Lines       28739    28751      +12     
==========================================
+ Hits        25082    25094      +12     
  Misses       3657     3657

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_tropomi_l2.py	94.05% <100%> (+0.24%)	⬆️
satpy/readers/tropomi_l2.py	94.87% <100%> (+0.58%)	⬆️

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[mraspaud]

LGTM