tetgen "litter" files

[ahmadia]

After any Proteus run where tetgen is executed, the following files are left lying around:

tetgen.ele  
tetgen.face 
tetgen.node

We should probably either clean these up by default or perhaps leave them in a temporary subdirectory (that the user can then inspect if for some reason a run has gone wrong).

They're not a huge annoyance, but I think it would be good form not to leave them lying around.

[adimako]

@cekees should not be too hard to do. I also recommend expanding the issue a bit and:

• Use a default value for the -D option (if not provided in command line)
• Write all files relative to the simulation run to this folder inc context options, mesh and output

[tridelat]

@adimako @cekees I think it is preferable to keep those files, and possibly in the same main folder as it is now, as they can be used for runs using the same mesh but different initial/boundary conditions. This is particularly convenient for larger mesh as mesh generation can take a while. Or we could have a way to specify a mesh file to use when initiating parun ...