Add latitude_bounds and longitude_bounds to tropomi_l2

[zxdawn]

Add latitude_bounds and longitude_bounds to tropomi_l2 reader.

KNMI researchers usually use these two variables to plot data. I've compared the differences of using longitude /latitude and latitude_bounds/longitude_bounds. Details are posted on my blog. The difference is ~ half pixel.

[coveralls]

Coverage increased (+0.001%) to 88.894% when pulling 48f47a6 on zxdawn:tropomi into ca8595f on pytroll:master.

[codecov]

Codecov Report

Merging #1084 into master will decrease coverage by <.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master    #1084      +/-   ##
==========================================
- Coverage   88.89%   88.89%   -0.01%     
==========================================
  Files         192      192              
  Lines       29657    29660       +3     
==========================================
+ Hits        26364    26366       +2     
- Misses       3293     3294       +1

Impacted Files	Coverage Δ
satpy/readers/tropomi_l2.py	94.11% <75%> (-1.01%)	⬇️

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[mraspaud]

Could you explain why this change to the code is needed ?

[zxdawn]

@mraspaud For example, lat_shape is 1 x 373 x 450 and the shape of latitude_bounds is 1 x 373 x 450 x 4. So, that if statement would ignore latitude_bounds and longitude_bounds.

[mraspaud]

LGTM