added generalFit2D.m and gaussConvolveStep function

generalFit2D takes any fittype and fits it to x/y data
gaussConvolveStep.m is a gauss convolved with a step functions
Merge branch 'general-fit-functions' into TYPHOON
added get_gel_lanes.m and necessary subfunctions

get_gel_lanes.m loads image (multiple channels possible), corrects
background, fits lane positions, calculates lane profiles and then
returns lane positions and lane profiles in a struct