ENH: Strain energy models

This MR adds the linear strain energy model that we had a flag for, but did not supports, as well as fixes a bug with the NeoHookean model that caused the model to be stiffer than intended. It also simplifies the implementation of the models using Eigen.

Also, new benchmarks were added for each model and I found that the performance was generally a bit faster than the original implementation when the code was simplified using Eigen, which is nice. :)

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)
• No new dependencies were added (or the new dependencies have been approved)
• Added test (or behavior not changed)
• Updated API documentation (or API not changed)
• Ran the CodeFormatEnforcer task and verified the license header is in all new files
• Added iMSTK examples for all new major features (if any)