`sampler` should sample along irregular CoordinateAxis

[cboulay]

Unlike windowing, sampler returns a list of AxisArray objects so it is possible for each object in the list to have different lengths along "time" axis, as well as different irregular intervals between time points. Thus, we don't technically need to wait until after the refactor from generator methods to class objects before we can implement this.

TBD when this will be done.