API change 2022 04 20 Pass CoordinateSystem object to forcing functions

PR312 introduces a CoordinateSystem object that is now passed to all forcing classes such as TPXOTidalBoundaryForcing. Previously the coord_system argument was a proj object, now it is replaced by a CoordinateSystem instance. For example, in the case of a UTM coordinate system one would do

import coordsys
coord_system = coordsys.UTMCoordinateSystem(utm_zone=30)

The new usage is demonstrated in the North Sea tidal model demo.