Discuss MorphRGrid handling of sparse grids

`MorphRGrid` uses linear interpolation to match two functions on to the same grid. While this works fine for dense grids, for sparse ones (e.g. Nyquist-Shannon), this can become very inaccurate.

We should either give the user a warning on grid sparsity (need discussion on what is considered sparse -- set a threshold), or see if there are alternative interpolation methods to use (e.g. WS interp).