Add elasticity example in fvmbook/chap22/inclusion.

This is a conversion of the example from Chapter 22 of Randy's book, based on what's in Clawpack 4.3.  Now it runs in PyClaw, using the solvers added in https://github.com/clawpack/riemann/pull/39/.

I also updated another book example (Burgers) and added a bit of error checking in Solution.__init__().