Creates a boundary structure for a mesh

  USAGE:

       vmcboundary = createBoundary(vmcmesh)

           Create a boundary structure with a minimal number of fields
           in order to run a simulation, i.e. a lightsource field whose length is
           equal to the number of boundary elements

        vmcboundary = createBoundary(vmcmesh, vmcmedium)

           In addition, create exterior_refactive_index so that
           photons experience no change in refractive index as they
           leave the medium

        vmcboundary_out = createBoundary(vmcmesh, vmcmedium, vmcboundary)

           Repeat the entries in vmcboundary so that the lengths
           of the arrays are equal to the number of boundary elements in vmcmesh.

        [vmcboundary, vmcmesh_out] = createBoundary(vmcmesh, ...)

           In addition, create a boundary element topology (BH) in vmcmesh_out

 INPUT:

       vmcmesh      - mesh structure, contains the geometry of the system

 OPTIONAL INPUT:

       vmcmedium    - medium structure, used to set e. g. optical coefficients
       vmcboundary  - boundary structure, used to set e. g. light sources

 OUTPUT:

       vmcboundary  - boundary structure

 OPTIONAL OUTPUT:

       vmcmesh_out  - vmcmesh complemented with boundary element structure

 SEE ALSO:

       https://inverselight.github.io/ValoMC/structures.html

 This function is provided with ValoMC