Bar width in units, retrieve bar peers directly

Fixes #443 and #338, i.e. uses now axis units for bar width/shift. This implicitly deals with multiple barplots created with hold on (thus fixes #338).

Also, I am getting rid of `countBarplotSiblings()` and retrieving the  bar group directly when plotting grouped. Octave 's property is 'bargroup' and Matlab's hidden property is 'BarPeers', which exists in both R2014a and b.

`Shift` and `barId` are calculated locally from the bargroup. This avoids popping up some properties into the `m2t` container.