Non watertight STL mesh for boundary faces

Hi

First, appreciate sharing this work on Github. It's an excellent library and solves real world problem.

I have a couple of questions when specifying boundary conditions.

1. Is it useful to support non-watertight STL mesh (which cuts the background mesh)? Current code assumes cutting STL mesh is watertight. I tried to relax this condition and use a simple STL representation of a face, but it ran into indexing issues. Reason for this is it will make it easier to work with CAD geometries, e.g. generate STL mesh from a CAD face and use it for boundary condition. Of course, using a thickened version of a face also solves this problem, however it adds additional complexity.
2. Do you see any issues with this approach of using separate STL mesh for BC's and in general can another STL mesh which represents a body attached to main geometry be directly used for STL mesh ? E.g. bracket attached to a fix plate, and both are separate geometries. This also makes it easier to setup BC's in some cases where it is extremely tedius to define faces for BCs. Instead its much easier to take another body for this.
Appreciate your suggestions.

Regards
SK

Start by tracing the current watertight-STL assumption and the indexing path mentioned in the issue. Determine whether non-watertight boundary faces and separate attached STL geometries can be supported without ambiguity, then define tests for those cases; no specific files or tests are named in the issue.

Written by the indexing model from the issue text.