API change 2022 03 23 Changes to forcing function arguments

PR #303 changed the behaviour of forcing classes such as TPXOTidalBoundaryForcing so that they accept a coordinate system, as opposed to a function transferring between coordinate systems. The same argument slot is used (the third argument in the case of TPXOTidalBoundaryForcing).

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