Added signal type selection

Made actions (menu entries) for changing signal's type. Also made helper
functions on SignalWrapper that will switch the geometry of signal and
navigator when switching from spectrum to image. This also helped to fix
some geometery sav/load issues that were previously.
Added auto-dtype for PCA

Automatically converts signal (in a background copy) to dtype float for
PCA.
Also moved fourier ratio to EELS category.
Added signal type filter utility class

Added class SignalTypeFilter which is meant to go as the selection
callback for actions. If the signal is of the correct type (checked by
isinstance) it will enable the action, if not, it will be disabled.