Deprecate Basemap dependency

[dnerini]

This PR removes the support for the Basemap module. As a consequence, all drawing of geographical features will be based on Cartopy only.

In detail:

• Add drawlonlatlabels option (closes Axis tick labels with lon-lat lines #14)
• Include additional geographical features (reefs, minor islands) when scale in ["10m", "50M"].
• Passing geodata without Cartopy installed silently fails, producing a georeferenced plot without any basemap (which is equivalent to pass plot_map=None).
• Add support for Geostationary and AzimuthalEquidistant projections.

See below for plots using pysteps example data:

[codecov]

Codecov Report

Merging #180 (6f1a15e) into master (b88b1b2) will increase coverage by 0.40%.
The diff coverage is 65.26%.

@@            Coverage Diff             @@
##           master     #180      +/-   ##
==========================================
+ Coverage   74.40%   74.80%   +0.40%     
==========================================
  Files         110      110              
  Lines        8802     8736      -66     
==========================================
- Hits         6549     6535      -14     
+ Misses       2253     2201      -52     

Impacted Files	Coverage Δ
pysteps/visualization/animations.py	5.68% <10.00%> (+1.08%)	⬆️
pysteps/visualization/motionfields.py	71.66% <46.66%> (-0.99%)	⬇️
pysteps/tests/test_plt_precipfields.py	88.88% <50.00%> (-0.40%)	⬇️
pysteps/visualization/utils.py	51.61% <63.63%> (+7.64%)	⬆️
pysteps/visualization/precipfields.py	66.29% <66.66%> (-2.52%)	⬇️
pysteps/tests/test_plt_cartopy.py	84.21% <83.33%> (ø)
pysteps/visualization/basemaps.py	81.81% <83.33%> (+32.39%)	⬆️
pysteps/nowcasts/sseps.py	87.52% <100.00%> (ø)
pysteps/tests/test_plt_motionfields.py	88.23% <100.00%> (+0.96%)	⬆️

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[RubenImhoff]

Nice work, @dnerini! Looks ready to go!

[pulkkins]

Nice work! I'm wondering should we still keep the plot_map option in the interface. In addition to cartopy, there are quite many map plotting tools that could be supported in future releases of pysteps.

[dnerini]

I'm wondering should we still keep the plot_map option in the interface. In addition to cartopy, there are quite many map plotting tools that could be supported in future releases of pysteps.

It's a good point, it would also improve back-compatibility and provide a clear way to not plot a basemap (plot_map=None). I'll make the change asap.

edit: done in e5fed04