Add inertia_matrix property to six_x_six schema

[deslaughter]

Add inertia_matrix to six_x_six structure

This PR adds the inertia_matrix field to the six_x_six schema as a duplicate of the stiffness_matrix field.

Impacted areas of the software

This change impacts IEAontology_schema.yaml and requires that the inertia_matrix field be part of the structure.

Additional supporting information

The inertia_matrix properties are the same as stiffness_matrix in the six_x_six structure. However, this field was not part of the schema, so when using the schema to generate C++ data structures, this field was missing.

Test results, if applicable

Test results are unaffected.

[deslaughter]

This is being addressed in the next version of WindIO, PR #87, so I'm closing this PR