Fixes to Interpolate, Normalize, Convolve. Addition of basinhopping to the fitter

Hi Wolfgang,

I have made some small changes to Interpolate, Convolve, and Normalize plugins. Comments are made for each commit.

I have also added basinhopping and all necessary changes for that. I did a quick test to make sure that it did not cause leastsq to fail and it did not fail for me. 

I am sure there are other small changes to be made, but I think this is a good start.

Nathan