Rename Elasticity -> Elastostatics?

[inducer]

@alexfikl What do you think?

If we do this, now would be the time while this doesn't yet have much in the way of users.

[alexfikl]

I think I would prefer something like KelvinKernel or NavierCauchyKernel instead? I'm not sure what's common in the BIE literature?

In general, I do agree that "elasticity" might be a bit too broad.