Add constant strain rate mode

[rcarson3]

I had a user reach out about needing constant strain rate for some of their exploratory studies. It seems like I could probably follow something like what was discussed in the comment thread at #26 (comment) .

It'll probably take a little bit of work to get a POC going, but I don't think it should be too awful with the changing BCs functionality now within the code.

As a first pass, the easiest version will be something like a forward eulerian version of things where we calculate the velocity conditions based on the previous time step nodal locations. After we get that looking, we can look at more expensive versions like ones that do an iterative approach to calculate at the end time step or mid-time step values.

Additionally, I plan on allowing this took work with meshes that don't have a min at origin. Also, I plan on requiring the user to define what degree of freedoms are free and which ones are constrained to the velocity gradient condition.

As part of this set of calculations, I figured I would also calculate the Eulerian strain for the user as we can easily get out the deformation gradient, and so that value isn't hard to get at all. I figure this would make it much easier for them when calculating post-processing values as they'll automatically have the "true" strain values. If they want the lagrangian strain tensor, they can just get that from the deformation gradient...

[rcarson3]

The setting essential boundary condition portion of things given the current implementation within ExaConstit will end up being trickier than I expected. Since I'd been contemplating redoing the BCs portion of things as part of #41, I guess I ought as well fix it here...