implement modifiers "flatten" and "ignore-case"

[marc-portier]

• flatten should smash down nested structures into keys --> thus accessing _.x.y.z via _["x.y.z"]
• ignore-case should make all access-keys lowercase or at least make accessing them case-insensitive ?

can we find a way to have that across all sets?
in fact does such beasts exist in py world already? (pandas?)

[marc-portier]

• starting to get a plan on doing this for xml via --> Missing element values with unwrap() py-xmlasdict#6
• would need to have small similar logic to do something similar for json stuff

But starting to doubt if ignore_case support is really such a good idea --> both json and xml input (and py dicts themselves) make a clear distinction -- it starts to feel wrong to smash those together at this level? Specially considering possible lists being mixed.... but then again adding this convenience inside pysubyt might be helpful in freeing users of the burden to do the cleaning themselves in pre-processing... hm need to think some more