Abstract interface to solve the inverse mapping problem

Add an abstract interface to solve the inverse mapping problem (R,Z(ix,iy) is known, find ix,iy(R,Z) for a collection of points).

There are many sticking points to consider. Right now map_mod also collects all blocks and all Fourier components to create a global domain on a MPI process. This permits ease of parallelization while it limits scalability.

The interface should accommodate both the native grid-block decomposition and fused-grid structures.

Concrete instantiations can be written for both unstructured- and structured-block grids.