add target_extent for mapdensity

[Chilipp]

This PR implements a fix for #43 (ping @victoria-cherkas). The quiver method of cartopy takes atarget_extent keyword that defaults to the map projection limits (which is by default global in the beginning).

        target_extent: 4-tuple
            If given, specifies the extent in the target CRS that the
            regular grid defined by *regrid_shape* will have. Defaults
            to the current extent of the map projection.

We did not specify this, which it is why the quiver collection has a global datalimit. This PR should fix this

closes #43

• [x] Tests added (for all bug fixes or enhancements)
• [x] Tests passed (for all non-documentation changes)
• [x] Fully documented, including CHANGELOG.rst for all changes

[codecov[bot]]

Codecov Report

Merging #44 (ece92c0) into develop (9b7dad4) will increase coverage by 0.14%. The diff coverage is 94.11%.

@@             Coverage Diff             @@
##           develop      #44      +/-   ##
===========================================
+ Coverage    78.41%   78.55%   +0.14%     
===========================================
  Files            6        6              
  Lines         1348     1357       +9     
===========================================
+ Hits          1057     1066       +9     
  Misses         291      291              

Impacted Files	Coverage Δ
psy_maps/plotters.py	85.51% <94.11%> (+0.11%)	:arrow_up:

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