add target_extent for mapdensity

This PR implements a fix for #43 (closed) (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 (closed)

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