xview_stepper implementation

[JohanMabille]

The current implementation of xview_stepper assumes a uniform step_size per slice, and adds it to the underlying iterator when stepping. This is somehow equivalent to a strided view.

However, if we want to implement slices such as "contains indices" or "drop indices" (i.e. slices that check that the given index is or is not in a given list), this implementation needs to be improved. For now, the only solution I see is to add a parameter to the step_size method of the slices, this parameter being the current index in the given dimension.

That implies tracking the full index in the stepper view (meaning additional memory allocation) and thus I wonder if going for an xindex_stepper could be more efficient.

[wolfv]

I am closing this issue as we've implemented this for the new islice