mpfit: changed gtol from 1e-10 to 1e-100
updated section "Curve Fitting" in the manual
added Fityk::get_function(name) to the API
added another test: tests/psvoigt.cpp
clean-up list of fitting methods

mpfit is now default,
6 NLopt methods are included,
a bit of renaming
use MPFIT to calculate cov.matrix and errors

(if custom lev-mar is used, errors are calculated using the old code,
just in case the default method doesn't work for some cases)

to avoid some conversions double is used instead of realt in a few
places