HGTD digi tool

As discussed during the USI meeting, I will start merging in the digi part for HGTD.

This first (out of probably three) MR contains the digi tool itself. Since @tawang is still working on the frontend tool, I am merging them in "reverse" order, first digi tool, then surfacecharge generator and lastly the frontend tool.

Since we talked in the recent HGTD related MRs about decreasing the number of fields in the classes and using writehandle etc, there is maybe some improvement to do, but I would like to not delay things too much.

For reviewers: IHGTD_SurfaceChargesGenerator related includes/variables are commented out to have the package compile, those will be uncommented in a following MR.

Tagging @cohm @dshope @tadej @ncalace @nstyles, would be happy about any feedback!