Add fitting option to plotting

Add fitting options to the Plot settings.
Various fitting functions can be selected.
Each dataset will be fitted.

Todo:
- [x] Nice display of the resulting function (perhaps in the div#results, so replacing the data preview table if it is there...), scientific notation.
- [x] ~~Perhaps make the fit always a line plot (no step and no points)~~ lines can be slower to plot, points are fine for now.
- [x] Only show the minus sign in powers, not the plus sign.
- [x] Fix polynomial fit

@jorianvo