Fix bug polaris total scattering normalisation for multi-atom unit cells

[RichardWaiteSTFC]

Description of work

There was a bug in the call to MaterialBuilder here https://github.com/mantidproject/mantid/blob/2ae50d1627844610269f2d8abae305fa68248811/scripts/Diffraction/isis_powder/polaris_routines/polaris_algs.py#L107 The build method requires unit cells with multiple atoms to have a number density set.

The number density is not actually used to calculate the cross-sections required for the PDF normalisation - so here I just set it to an arbitrary number (1).

There is no associated issue.

Report to: Helen Playford

To test:

(1) Unzip these files somewhere and update the script below with the path. Also update the paths in the file polaris_config_example.yaml PR34144Files.zip (2) Run this script without error

from mantid.simpleapi import *
import matplotlib.pyplot as plt
import numpy as np
from isis_powder import polaris, SampleDetails


config_file_path = r"C:\Total Scattering\polaris_config_example.yaml"
polaris = polaris.Polaris(config_file=config_file_path, user_name="test", mode="PDF")

sam = SampleDetails(height=4.6,radius=0.29855,center=[0,0,0],shape='cylinder')
sam.set_container(radius=0.3175,chemical_formula='V')
sam.set_material(chemical_formula='Pb-Sn-F4',
number_density=0.059, packing_fraction=0.5)
polaris.set_sample_details(sample=sam)

polaris.create_vanadium(first_cycle_run_no="98532", multiple_scattering=True, do_absorb_corrections=True, mayers_mult_scat_events=1)
polaris.focus(run_number="98533", input_mode='Summed', do_absorb_corrections=True,multiple_scattering=False,
              do_van_normalisation=True, van_normalisation_method="Absolute",
              keep_raw_workspace=False, mayers_mult_scat_events=1)

# q_lim from script on https://github.com/mantidproject/mantid/pull/28447
q_lim = [[2.5, 3, 4, 6, 7], [3.5, 5, 7, 11, 40]]
polaris.create_total_scattering_pdf(run_number="98533", merge_banks=True,q_lims=q_lim, 
                                    freq_params=[1.0], pdf_type="g(r)", merge_banks=True,)

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