Extend PoldiFitPeaks2D so parameters can be tied

[MichaelWedel]

This fixes #12725.

Testing information
Code review. New option is covered by system test and unit test, documentation has been updated. For manual testing you could take this modified example from PoldiDataAnalysis:

# Load POLDI data
PoldiLoadRuns(2013, 6903, 6904, 2, OutputWorkspace='poldi', MaskBadDetectors=False)

# Create Silicon peaks
PoldiCreatePeaksFromCell(SpaceGroup='F d -3 m', Atoms='Si 0 0 0 1.0 0.01', a=5.431,
                         LatticeSpacingMin=0.7,
                         OutputWorkspace='Si')

PoldiDataAnalysis(InputWorkspace='poldi_data_6904',
                  ExpectedPeaks='Si',
                  MaximumPeakNumber=11,
                  PlotResult=True,
                  ProfileFunction='PseudoVoigt',
                  TieProfileParameters=True,
                  OutputRawFitParameters=True,
                  OutputWorkspace='result')

If TieProfileParameters is True, the plot should show a reasonable fit. If you change the value to False, the result will look terrible. Looking at the workspace poldi_data_6904_raw_fit_parameters should contain the same value for all Mixing parameters (if ties are enabled), it should be close to 1 (0.86 +/- 0.065) on my machine.

[FedeMPouzols]

All looks good and the manual examples seem to work well. I got Mixing: 0.887033 +/- 0.0652029 (on debian). The only nitpicky issue that I could catch is that the progress bar doesn't seem to follow the progress of PoldiDataAnalysis? Anyway it doesn't take that long and the logs keep telling you what's happening.

The plots look fine and wrong as expected. With the tie option on:

With the tie option off: