ADD Figure class abstraction for experiment.Pulling

Used to provide a bit nicer interface to figures
used inside of these objects. Not sure yet if it
will end up as a real class or simply for internal
use inside of experiment objects.

Specifically, Figure can at least tell you if it exists (is visible)
and also save itself to file.
Merge branch 'master' of github.com:cfperez/smbanalyze