GRHD: more specific boundry conditions [only incorporated for FV+slope_limiter]

This adds new options under GRHD.bc:

• tov_reflective_origin: impose reflection symmetry at coordinate origin, impose vacuum values at outer bdry
• tov_symmetric_domain: impose vacuum values at both domain boundary ends

Also streamlined the setup of the bdry values for FV. While doing so I realized that the inflow bdry in the Bondi test was wrongly treated. There we cannot extrapolate interior data, nor just copy what we have in the interior, unless we are very far out (which in my test we aren't). Instead, what we do now there is use the initial data, which is the analytical solution and extrapolate that. This improves the behavior of the solution to no longer drift after the first time step. Nice.