Add convex hull option to quality mesh
The quality mesh currently only supports the 'Shrink to segments' mesh boundary option.
The custom boundary option is tracked in #16 (closed), but there are some use cases for which 'Keep convex hull' might also be handy for use in quality meshes.
Firstly, non-wetted cells are cheap during the simulation process - for smaller meshes or tests, the outer boundary is not critical. Further, this allows the generation of meshes from point data alone, assuming a sufficiently small global max-area constraint has been specified.