Added method to define zero and non-zero boundary conditons on indivi…

[JohnDN90]

This allows dirichlet boundary conditions, both zero and non-zero, to be applied on individual degrees of freedom. The formatting for defining BCs is similar to that of Nastran. The new class accepts three vectors which defines the nodes the BC is applied to, the DOFs the BC acts on within those nodes, and the value for each of those DOFs.

This was implement a long time ago (~10 months?), but was never moved from GitLab to here. This pull request moves this into the GitHub repo.