Fixing name for dispersity parameter for multishell modells

[dehoni]

Description

onPolyComboIndexChange takes the parameter name as thickness[n] instead of the correct value, e.g. for shell 2 this should be thickness2, such that sasmodels can handle it. A fix needs to ensures to pass on an expanded parameter name like thickness2 to kernel_module.set_dispersion instead of the thickness[n] which triggered the ValueError.

Fixes #3741

How Has This Been Tested?

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Documentation (check at least one)

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Installers

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Licensing (untick if necessary)

• The introduced changes comply with SasView license (BSD 3-Clause)