* Implement CorotatedModel and math helpers.

- Implement the fixed corotated model from Stomakhin, Alexey, et al.
 "Energetically consistent  invertible elasticity." Proceedings of the
 11th ACM SIGGRAPH/Eurographics conference on Computer Animation. 2012.

- Implement a few matrix calculation helpers to facilitate
 CorotatedModel.

- Refactor utility functions for St-Venant type constitutive models
 into free functions.

- Refactor LinearConstitutiveModelTest to test for both
 LinearConstitutiveModel and CorotatedModel.