mesh bp: support embedded ghost cells for structured meshes

[gzagaris]

Codes that use structured meshes which contain ghost cells need to specify the ghost regions when publishing their data to the mesh blueprint.

One common way to do this is to specify a "real extent".

This issue proposes adding an optional "real_extent" field to the implicit topology, for example, something like this:

topologies/topo/coordset: “coords”
topologies/topo/type = “structured”
topologies/topo/elements/dims/{i,j,k}
topologies/topo/elements/real_exent/{imin,imax,jmin,jmax,kmin,kmax} {optional}
topologies/topo/elements/origin/{i0,j0,k0} (optional, default = {0,0,0})

[cyrush]

@gzagaris we should chat about this, first pass at ghost support will be a field with a bit field. This is needed even in some cases for structured grid b/c not all ghosts have the same role. Some are duplicated, some are just plain garbage and need to be excluded.

[gzagaris]

@cyrush sounds good, yes let's chat about this.

[nselliott]

I am interested in this as well.

[gzagaris]

@nselliott I sent you an invite.