Upstream boundary accessors for 1D simplicial sets

[lukem12345]

Currently, this package exports helper functions which access the indices of boundary simplices for 2D simplicial sets.

However, boundary accessors for 1D simplicial sets are not exported. This is typically because the 1D simplicial sets used in simulations are either lines or circles. For the line mesh, grabbing the boundary indices is simply a matter of calling [begin] and [end] on the co-chain of interest.

For other meshes, i.e. those with multiple connected components, [begin] and [end] will not suffice.

So, we should upstream some helper functions:

boundary_verts =                                    
  findall(x -> x < 2,
          StatsBase.counts(vcat(sdE[:∂v0], sdE[:∂v1])))       
boundary_edges =
  union(reduce(vcat, incident(sdE, boundary_verts, :∂v1)),
        reduce(vcat, incident(sdE, boundary_verts, :∂v0)))

The star operator could be used as well:

boundary_edges = mapreduce(v -> star(sE, v)[2], vcat, boundary_verts)