MNT: walk bases in mro order
MNT: walk bases in mro order

This will construct the `_sig_attrs` in the order the mro is consulted
to look up functions.  This means that

  - iterating _sig_attrs you strictly see signals from parents
  - first (reverse of mro lookup of functions)
  - if we allow over-riding of components then they will appear in the
    iteration with the lowest sub-class on the tree that defined it

dchabot/ophyd,dchabot/ophyd