[0.20] Draft: allow items to be skipped in draft array

[davidosterberg]

Implemented for Array, Polar array, and Circular array.

Motivation for feature:
Often when you make pattern (e.g a bolt pattern), there will by one or two items that should be skipped for some reason.

Forum discussion:
https://forum.freecadweb.org/viewtopic.php?f=23&t=55459

If you consider this interesting, I will add support for the other Array types.
At a later stage this can also be exposed in the GUI

Advanced indexing:
Skip expressions are expanded in a python/matlab like way

    Example 1: skip_expression = 1:5
        Will be expanded to [1, 2, 3, 4]

    Example 2: skip_expression = [2:5, 9]
        Will be expanded to [2, 3, 4, 9]

    Example 3: skip_expression = [1, 3:7:2, 9]
        Will be expanded to [1, 3, 5, 9]

[davidosterberg]

There is already a way (perhaps not so discoverable) to skip items in draft array. Simply expand the array, the hide the item you don't want. Then the array can be un-expanded. Therefore I close this PR.