Check that material model supports elasticity when it is enabled

[anne-glerum]

See #4985. When a material model does not support elasticity, but elasticity is enabled, Throw a helpful exception.

I left in the AssertThrow in elasticity.cc that checks that Enable elasticity is set to true (so it checks the reverse of the new AssertThrow).

For all pull requests:

• I have followed the instructions for indenting my code.

For new features/models or changes of existing features:

• I have tested my new feature locally to ensure it is correct.
• I have created a testcase for the new feature/benchmark in the tests/ directory.
• I have added a changelog entry in the doc/modules/changes directory that will inform other users of my change.

[bangerth]

Yes, nice!

[bangerth]

Might want to squash the 5 commits into one :-)