Setting discontinuous initial conditions

As far as I can tell, if an initial condition is discontinuous across an element interface, it's ambiguous what value the node on the interface will be set to. It would be nice if there was a way to set discontinuous initial conditions in a non-ambiguous manner, or at least an elixir where `compute_coefficients!` is overloaded to demonstrate this process.

The issue does not name a file or test. Start by locating compute_coefficients! and reviewing how initial conditions at element interfaces are handled; done means discontinuous initial conditions have an unambiguous setup, or a clear example demonstrates the process.

Written by the indexing model from the issue text.