Use sp.limit instead of subs for default R/C...

Use `sp.limit` instead of `subs` method when substituting default `R` and `C`
values in computed transfer functions.

By using `sp.limit`, we avoid issues when, e.g., `R` is given a value of
infinity.  Specifically, this allows us to solve for `V2` when fitting the
impedance measurement parameters.

Other edits
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 - Do not use any `plt.*` functions to plot impedance stats, since this causes
   issues when plotting to a figure on a GTK canvas.
  * Specifically, although the plots are drawn appropriately on the GTK canvas,
    an empty `matplotlib` figure window is also launched.  By removing calls to
    `plt.*` functions, we avoid this behaviour.
Add impedance calibration assistant/wizard

 - Add overview figure to feedback calibration wizards.