Extend QLData workflow

For qldata workflow we should add an option to fix a peak width to a known value (so for a single peak the FWHM is not a free variable, for 2 peaks there is 1 FHWM variable etc.).

will need to update the workflow to use something like this
```
func.add_single_lorentzian()
if func._N_peaks == 1:
func.set_func_guess(value)
func.set_func_bounds(lower, upper)
return func
```