Add not to indexing

[bkamins]

This is not finished yet and I will add a list of changes it does.
In general I add Not for indexing + remove deletecols! as it is obsolete with Not + clean up several related methods.

[bkamins]

This PR should be good to have a look at.

Notes:

• it supports Not for row and column indexing
• it does not support Not in setindex! (this should be done in a separate PR that cleans up setindex! that should be merged before the next release)
• in the long run all codes related to indexing should be refactored to make the design simpler I think, but this can be done after 1.0 release as discussed earlier

Test on nightly fail because of InteractiveUtils.jl not-found error (I did not investigate into it but I think this is unrelated).