Make Function version of Slice

[dietzc]

Currently, slice needs some RAI<I> and RAI<O> and an UnaryComputer<RAI<I>,RAI<O>> in addition to the fixed axis. We can add a version which takes a Function<RAI<I>, O> and return an RAI<O> which has the dimensionality of the axis we were iterating. For example X,Y,Z,Time with fixed X,Y,Z axis would return a 1D RAI<DoubleType> if we want to compute the mean for each cube.

[hadim]

@hadim

(just to add me to the notif)

[stelfrich]

This has also come up in #461.

[dietzc]

I wouldn't address this before we have the new scijava-ops stuff in place.