33 elasticity for new interface 1

[srosenbu]

resolves #37 , resolves #33

This PR also introduces handling of history variables, but this is not tested yet. Use with caution

Todo

• test for uniaxial strain
• test for plane strain
• test for plane stress
• test for full 3D
• docstrings

Edit: I added the examples directory to the pytest configuration, therefore tests for constitutive models can go into their respective directory, instead of the tests directory of the package itself

[srosenbu]

@saifr68 , @pdiercks , I think the PR is ready for review. I still need to add some docstrings and the tests are currently only some sanity checks, but they all pass. The larger test problems whcih were suggested to me can come later as an exampe for the documentation.

[pdiercks]

Hi @srosenbu ,
I have mostly minor suggestions and questions.
I like what you did here, nice work !!