get rid of HydroSystem<problem_t> in front of density_index

**Describe the proposal**
Since hydroFirstIndex is always 0, the six hydro components are always 0-5 and will never change from problem to problem. So, why not remove `Physics_Indices<problem_t>::` in front of all hydro indices?

**Describe alternatives you've considered**

**Additional context**
Add any other context about the feature proposal here.
