FEATURE Pulling.fitFEC() for automatic offset alignment

fitFEC() method of experiment.Pulling objects iteratively fits
a WLC to data range specified until the apparent force offset
is < tolerance. In doing this, it also recalculates the extension
from the trap separation and force/bead displacement.

Also updated some defaults: Lc1 = 1 nm and K1 = 1600 nm (ssDNA)