Moved contact point detection from plugin.fit -> plugin.vclamp.

Moved contact point detection from plugin.fit -> plugin.vclamp.

I haven't translated the two algorithms brought over from fit yet, but
I have added my own bilinear fit approach:
  _find_contact_point_wtk

Also:
* vclamp.scale gets access to hooke for the command list and
  command._run(hooke, ...)
* vclamp.scale functionality broken out into SurfaceContactCommand and
  ForceCommand.
* 'surface z piezo (m)' -> 'surface distance (m)'
* Actually set params['curve'] for flatfilt and convfilt FilterCommand
  subclasses.  Previous implementation just banged away on the default
  curve, regardless of the curve option passed to the subclass' _run.