Add piecewise constant interpolation (zero order)

Suggestion:
In the ipolate proc, add another type of interpolation that implements zero order interpolation, aka, piecewise constant interpolation.
I know this could be done in custom code, however this way it would be much easier to implement and use.
Also, make possible so it could be centered or backwards looking only.

Described in
https://en.wikipedia.org/wiki/Interpola ... erpolation

---