Added tagging of parts within assemblies

[jmwright]

@shimwell This is a first attempt at adding volume tagging to the mesh exporter. I'm looking for some feedback. There are two things to note.

1. The current tagging method uses the name of the part in the assembly to tag the volumes. Should explicit tagging be used instead (or in addition to)?
2. The feature is currently on all the time, causing the resulting .msh files to have extra physical groups for the part volumes. Will this cause problems? If so, I can make it a parameter.

[shimwell]

use of name is fine with me. These names have to be unique so I would perhaps rename them later in the workflow.

For example there might be volumes called magnet1, magnet2, magnet3 but all of these would be made from the same material. So later in my workflow I would map to use a single material name.

But I think use of name is good with me.

Always on behavior and extra physical groups is also fine with me.

[jmwright]

Ok, I'll leave this open for two days or so, and then merge if there's no other comments.

[jon-proximafusion]

@RemDelaporteMathurin @jhdark does having tagged volumes in the gmsh object help your workflows for FESTIM

[RemDelaporteMathurin]

@jon-proximafusion well I believe this would help yes. I believe so far I've always tagged volumes and surfaces manually in GMSH with selectors (ie. is the surface near x=3, etc).