Add a bunch of two-state problems to problems.lua
Add outflow boundary conditions to grav1d

For uniformity, we also move the time update flag to fish.h, along with
the boundary conditions flag.

There is a mysterious behavior where sometimes PLM reconstruction looks
like it's doing PCM. Putting printf's in the _plm function shows that
it's indeed being called. For some reason, rebuilding reconstruct.c
appears to fix the problem when it happens. Valgrind does not report
anything suspiscious.