Include auto-amp gain in read/write config methods
Reduce impedance sampling time to 5ms...

Reduce impedance sampling time to 5ms, but collect 60 samples.  The goal of
this change is to help measurements stabilize.

TODO
====

 - Determine if 60 samples are actually required.  This might have been an
   issue of measuring the wrong analog input channel by enabling auto-aliasing
   when the auto-aliasing hardware filter was not connected.  Note that
   unconnected analog inputs are floating, and thus, may correlate to the state
   of other analog input pins, though with irregular behaviour.
Only use divergent colormap if some values are -ve
Fix frequency labels on impedance plots
Only use GTK threads enter/exit inside callback

Fix freezing GUI by only using GTK threads enter/exit inside callback
functions.  When called within the main program thread, the threads lock may
already be acquired in which case dead-lock will occur.
Do not save results by default

 - Do not save results by default.
 - Rename `save*` methods to `to_hdf`.
  * Inspired by `pandas` serialization method naming scheme.