Make KPAFM work with general bias voltage

[yakutovicha]

No description provided.

[mondracek]

The bias range has to be in the form Vmin = -V, .... , V0 = 0, ... Vmax = +V for KPFM to work. That is, the voltage samples must be positive and negative, symmetric with respect to zero, and one of the voltages has to be exactly zero. Otherwise, KPFM outputs a nonsensical LCPD, without warning. I want to remove this restriction.