Multi fit copy incr

Created by: jtrebosc

This is a new feature. When using fit all button one can check a button that will copy the parameter set step by step. Useful when fitting a slowly evolving parameter as the last row will start fitting from last -1 fitted set instead of first row set. Convergence is faster. I updated the Reference Manual. I made a tutorial that I will submit for a pull request soon. @smeerten and @wfranssen could you have a look ? Thanks