feature: easily create dis/v/u from grid / grb

[wpbonelli]

Suggested by @cneyens. It's a bit awkward right now to create a discretization object from a grid and/or binary grid file, you need to manually shuffle the parameters over AFAICT. This deserves some convenience (class-)methods or functions to convert between the grid representations.

Also, deprecate MfGrdFile.bot and alias it to botm for consistency.

Originally posted by @wpbonelli in #2655 (comment)