Multi-material restart

Adding integration point output for the solid material's internal state variables in the SmallDeformation process. This required extensive refactoring of the solidMaterialInternalVariablesToIntegrationPointWriter function to work with multiple materials each containing different internal state variables.

1. Feature description was added to the changelog
2. Tests covering your feature were added? Test modification and a restart test.
3. Any new feature or behavior change was documented? Restart works now as expected, no change required