MTF.py now does polynomial fits.

Introduced function based on code from http://is.gd/q18xED to perform a polynomial fit
Plotting the polynomial fit in a fourth row of the plot, so we can calculate the MTF @ Nyquist from this
much nicer plotting of additional data
Added polynomial fit to MTF_reader

Removed unnecessary constraint on range for LSF and MTF in the final plot
Added calculation of polyfit and display of this fit in the final plot