Adding TypeTensor<T>::inverse() code from @dknez.

Not tested with complex numbers...
Adding unit test for TypeTensor::inverse().
Ran bootstrap.
Use TypeTensor inverse code in FEM system ex2.
Replace 3x3 hand-coded inverse in fe_map.C
Use 'double' to avoid issues with 'Real' and finite precision literals.
Merge pull request #421 from libMesh/inverse_3x3

Add TypeTensor::inverse() and unit test