Fix for projections using a spherical earth

[mpvginde]

Some projections use a spherical earth. This is indicated in the proj4-string by either
the +R=... identifier or
by setting the +a= and +b= equal.

In both cases the parse_proj4_string returns a dict with the R-key, but this key is not checked by proj4_to_cartopy resulting in the default WGS84 globe for the cartopy CRS, giving small differences in the locations of the gridpoints.

This is now fixed by explicitly checking for the R-key in proj4_to_cartopy and defining the correct globe keywords.

I needed this to take advantage of the plot_precip_fields function for plotting precipitation coming from the ALARO NWP model.

[codecov]

Codecov Report

Patch coverage: 33.33% and project coverage change: -0.02% ⚠️

Comparison is base (57ece43) 83.34% compared to head (8856d5c) 83.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #336      +/-   ##
==========================================
- Coverage   83.34%   83.33%   -0.02%     
==========================================
  Files         161      161              
  Lines       12355    12358       +3     
==========================================
+ Hits        10297    10298       +1     
- Misses       2058     2060       +2     

Flag	Coverage Δ
unit_tests	83.33% <33.33%> (-0.02%)	⬇️

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Files Changed	Coverage Δ
pysteps/visualization/utils.py	56.42% <33.33%> (-0.51%)	⬇️

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[dnerini]

Hi @mpvginde thanks for the contribution! @pulkkins do you want to take this?