support custom projections

Hi,

This small pull request is only a slight rearrangement of relevant code in classes MaplyViewController and MaplyCoordinateSystem, to allow support for custom projections.

With this change one can subclass MaplyCoordinateSystem and MaplyViewController to use a custom coordinate system in the 2D view.

The MaplyCoordinateSystem-derived class would have a custom init method which calls initWithCoordSystem: from the base class.

The MaplyViewController-derived class would have a custom loadSetup_view method, which is identical to the base class method except that it uses a custom CoordSystem and CoordSystemDisplayAdapter.

For this contribution I am choosing Transfer Ownership.  Ownership is transferred to Mousebird Consulting Inc.

Thanks
Ranen