Allow nil as wildcard in elementAt()

[LFSaw]

something like

elementAt(\kn, nil, 5)

would be great to have

[adcxyz]

in republic:
key for 'where' param can be
single name,
index,
list of names,
and 'all'
then with nanokontrol1,
x.elementAt(\sl, \all, 8) could used to add
action to set mastervol

[adcxyz]

this could be addressed nicely in NamedList, see #211

[LFSaw]

I am for the implementation with NamedList.
Why \all, if there is nil?

[adcxyz]

for semantic clarity ;-)
nil could also be a variable that was not assigned by accident,
and then unexpected things would happen.

[adcxyz]

has been working for a while in desc_reorg:
m = MKtl(\x, "korg-nanokontrol");
m.elAt([\kn, \sl], 1, [0, 1]).postcs // knobs and sliders 1, 2 of scene 1
m.elAt(\sl, nil, [0, 1]).postcs; // first two sliders of all 4 scenes
m.elAt(\sl, \all, [0, 1]).postcs; // nil and \all work as wildcards.