Tilt + Stretch Stack

[z3tt]

Might be a good idea to add handling of raster stacks which would remove the need to run the tilt_map() function and to (mis)specify the arguments for each raster?

A proposed solution would be a tilt_stack() function where users specify the usual arguments but those are then applied to all landscapes (e.g. y_shift = 30 would mean a sequence of 0, 30, 60, 90etc as shift values for each raster).

[marcosci]

approved! :D