Copy Params to Format/Save Params to File throws error for Multiplicity models

[smk78]

Describe the bug
Whilst using the Unified_Power_rg model, User JamesD observed that trying to 'Copy params to Format' or 'Save Params to File' throws errors and does not work. The bug is easily to replicate.

15:23:15 - ERROR: Traceback (most recent call last):
  File "sas\qtgui\Perspectives\Fitting\FittingPerspective.py", line 120, in onExcelCopy
  File "sas\qtgui\Perspectives\Fitting\FittingWidget.py", line 3891, in onCopyToClipboard
  File "sas\qtgui\Perspectives\Fitting\FittingUtilities.py", line 806, in formatParametersExcel
IndexError: list index out of range
15:23:26 - ERROR: Traceback (most recent call last):
  File "sas\qtgui\Perspectives\Fitting\FittingPerspective.py", line 214, in onParamSave
  File "sas\qtgui\Perspectives\Fitting\FittingWidget.py", line 3896, in onCopyToClipboard
  File "sas\qtgui\Perspectives\Fitting\FittingUtilities.py", line 806, in formatParametersExcel
IndexError: list index out of range

To Reproduce
Run SasView and load up the Shape Independent > unified_power_Rg model
Compute the model
Attempt to Edit > Copy Params to Format/Save Params to File

Expected behavior
The model parameters should be exported. Though slightly different, this functionality works in 4.2.2

Workaround
Edit > Copy Params does work, but the format is not intended for the same use that Copy Params to Format/Save Params to File are

SasView version
Reproduced in 5.0.5, 5.0.4 & 5.0.3 so appears to be a general bug in 5.x

Operating system (please complete the following information):
x64/W10

[smk78]

On further testing, the same errors are thrown by the onion model, so it probably affects all multiplicity models.

[krzywon]

After some testing, the error is thrown for most/all multiplicity models. Normal parameters are lists of 7 values of the structure ['name', 'SelectedForFit', 'Value', 'Error', 'Min', 'Max', ()] but the multiplicity values like n of the multi-core-shell models are lists of 3 values: ['n', 'error', 'value']. The excel and latex export methods use indices that the multiplicity parameters don't have, causing the error.

[lucas-wilkins]

Explicit data structures would be nice

[smk78]

Also see #1106

[butlerpd]

Should be closed by the merge of #2647. Closing manually