Improved Mercator projection

This PR constrains the scope of #316, focussing simply on adding an improved Mercator projection without all the excess work around making the tests pass.

Instead the new Mercator class is designed to emulate the outgoing Mercator class by allowing certain kwargs (notably passing a globe with a semimajor axis of math.degrees(1) and setting both max_ and min_latitude to 85 degrees N/S) to be supplied to it. Instances of the Mercator Projection as used by the tests have been updated suchlike.