Restored linear-fitting option to surface bumps.

Restored linear-fitting option to surface bumps.

Now you don't have to use quadratic fitting if you don't want to.  This
mitigates problems associated with poorly defined contact-kinks, since
it's not a good idea to focus on the post-kink slope if you don't have
a good idea of where the kink actually is.  The limited_linear model
retains the non-contact and high-voltage-rail flat-line portions, with
a linear contact regime between the two.

Additional minor changes include:
  * Average deflection printout from vib_analyze in TEXT_VERBOSE mode.
    To make it easier to demonstrate that variance increases as the mean
    deflection deviates further from zero.
  * Corrected usage string in bump_analyze.py which had been out of date
    before.