Refactor Elasticity

Finite Elasticity equations are broken and are not tested. We have to solve this major issue by the following steps:

• Define a set of tests (from Wriggers, Non-Linear Finite Elements).
• Group the tests on static / dynamic / elastic / mixed etc.
• Write a Finite Elasticity Assemble which can be tested given the test framework
• Implement ALL Elasticity Tests
• Add the N-benchmark Active Strain tests
• Implement an active strain elasticity
• Implement a mixed elasticity