Don't include magnetism parameters in pure python models

[pkienzle]

Description

Use the model info interpreter from sasmodels.modelinfo rather than calling the internal modelinfo.make_parameter_table directly.

Fixes sasmodels #732

How Has This Been Tested?

The new tetrahedron pure python model loads without magnetism support, and the exist cylinder model loads with magnetism support.

Review Checklist:

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Documentation (check at least one)

• There is nothing that needs documenting
• Documentation changes are in this PR
• There is an issue open for the documentation (link?)

Installers

• There is a chance this will affect the installers, if so
  • Windows installer (GH artifact) has been tested (installed and worked)
  • MacOSX installer (GH artifact) has been tested (installed and worked)
  • Wheels installer (GH artifact) has been tested (installed and worked)

Licensing (untick if necessary)

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