Expose fitting parameters to python

[RichardWaiteSTFC]

Is your feature request related to a problem? Please describe.
POLDI has the wavelength dependence of the flux as a fitting parameter in the IDF - we need to access this from python

mantid/instrument/POLDI_Definition_2015.xml

Lines 42 to 47 in 627ae2f

	<parameter name="PoldiSpectrum:WavelengthDistribution" type="fitting">
	<!--
	This is the POLDI neutron wavelength distribution. The intensities
	are given such that Mean(I) = 1.0.
	-->
	<lookuptable x-unit="Wavelength">

from mantid.simpleapi import *
ws = LoadEmptyInstrument(InstrumentName='POLDI', OutputWorkspace='ws')
source = ws.getInstrument().getComponentByName("source")
print(source.hasParameter("WavelengthDistribution"))  # True

but try to access it e.g.

source.getNumberParameter("WavelengthDistribution")
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
Cell In[11], line 1
----> 1 source.getNumberParameter("WavelengthDistribution")

RuntimeError: Wrong type of parameter.

I can't see a getFittingParameter or similar function - this is how it's accessed in C++

mantid/Framework/SINQ/src/PoldiUtilities/PoldiSourceSpectrum.cpp

Lines 83 to 84 in 627ae2f

	Parameter_sptr spectrumParameter =
	instrumentParameterMap->getRecursive(&(*source), "WavelengthDistribution", "fitting");

It seems like this is not exposed to python

mantid/Framework/Geometry/src/Instrument.cpp

Line 137 in 627ae2f

ParameterMap_sptr Instrument::getParameterMap() const {

You can also look at setMatrixWorkspace which parses the Parameters .xml file to initialise parameters of fitting functions.

Describe the solution you'd like
Expose fitting parameters (such as the flux in POLDI, but also peak profile parameters etc.) to python