implement inflow BCs for the flame

[zingale]

PR summary

This will allow us to inflow fuel in through the right boundary to hold the flame roughly stationary. Better still, it will hopefully remove the influence of an outflow BC on the flame propagation.

PR motivation

PR checklist

• [ ] test suite needs to be run on this PR
• [ ] this PR will change answers in the test suite to more than roundoff level
• [ ] all newly-added functions have docstrings as per the coding conventions
• [ ] the CHANGES file has been updated, if appropriate
• [ ] if appropriate, this change is described in the docs

[zingale]

I don't think this is correct yet. We need to fill the ghost cells such that when we interpolate to the boundary we get the inflow velocity on the interface. So this means I need to use the proper stencil there. I can either do this in problem_initialize_state_data.H or by modifying the stencils in the interpolation.