More utility functions/a utility function file

[rikingurditta]

The assignments typically require a little bit of code reuse, but there is no file in the Cmake project for us to define our own functions. As a result sometimes we need to copy/paste code and copy/paste updates/fixes.
For example, the deformation gradient as well as the B and N matrices in A4 need to be calculated in both dV and d2V.

A file for our own functions (or assignment files for these things) would be really nice in future iterations of the course.