Transitioned severity to Command-format, also added Command._get_*()

Transitioned severity to Command-format, also added Command._get_*()

The old
  .requires_*
thing was rediculous.  The new ._get_*() callbacks allow the caller
to provide a means for getting the expensive structures, which the
command can use, or not, as required.  This will also make it easier
to implement the completion callbacks.

The callbacks should probably have matching .set_*() methods, to
avoid the current cache tweaking
  cmd._storage = ...
etc.  But that can wait for now...