create FaceCenterData class about pyro2

[zingale]

there is now a FaceCenterData2d class. But we need to subclass ArrayIndexer as well. In particular, .v() should give the ng+1 elements corresponding to the left and right boundaries in the face-centered direction, and we need to implement boundary conditions.

[zingale]

also an issue... we rely on the Grid2d.norm() function to compute norms, through the ArrayIndexer, but this won't know about data centerings. We should move the norm stuff out of the Grid2d class.

[zingale]

this is implemented in the mhd branch