Changed the internal workings of _transform_vector.

Functional changes include:
  * define delta based on the native projection extent
  * check for out-of-range values in the x and y directions,
    where the range is determined from the source projection
  * signature changed, now requiring coordinates

Other changes made for clarity of the code and documentation:
  * variable name changes
  * docstring typos fixed
  * added argument descriptions in docstring
Make the vector transform method part of the CRS class.

In summary:
* Removes the _transform_vectors method from GeoAxes
* Added a transform_vectors method to CRS
Make integration test plots more interesting.

These tests now use more complex vector fields and are coloured
according to the magnitude of the vectors.
Added missing docstring to CRS.transform_vectors.
Modify CRS.transform_vectors to take matching shape 1d or 2d inputs.

This modification brings the usage of CRS.transform_vectors more
in-line with that of CRS.transform_points.
Added a test for CRS.transform_vectors.
Added a placeholder entry to what's new.