Fluent API for arbitrary grid motion with interferograms

[brandondube]

The new interface for interferograms requires the user to explicitly pass the mask, and provides the x,y,r,t variables in class with the positional state. This makes it significantly easier to use offset masks. The geometry module includes shifted versions of most shapes, but it may be worthwhile to add a interf.shift((dx,dy)) or interf.shift(dx, dy=None) method to move the grid.

It would be most efficient not to shift the dense grid. Benchmarking would need to be done to see whether optimize_xy_separable, shifting the vectors, and regridding is faster than shifting the grid. If the grid has not yet been computed, then the initialization could be modified, which will be cheapest overall.