Read GMSH mesh file and create BoundaryFeature

[markstock]

As a step toward the hybrid solver, we need to be able to read boundary (only) elements from an .msh file as output from GMSH and turn them into a BoundaryFeature. For starters, we can keep the elements all linear and 1st order.

For this, a new BoundaryFeature subclass will need to be created for "user-defined" outlines (see #56). Once this is functional, we can expand the msh-reader to include the volume elements (quads first) and then high-order volume elements.