Update LengthIntegrator to be consistent with new developments from 21.9
LengthIntegrator is used to make material profile plots such as those here:
https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PLOTS/ITK-2019-001/fig_01.png
In 21.9 this was updated by @npetters to use a TTree for storing the information extracted, rather than producing a large number of histograms, for ease of use and to avoid the large memory overheads of having a histogram per material/geometry element found. This MR ports across those updates, and removes much of the old implementation.
This new implementation relies on lists of material assigned to categories "by hand", and implements the categorization as appropriate for the ITk. Other use cases would need this to be adapted as appropriate. Since ITk is the main client of this tool, this was checked with @jchapman, who agreed that this could proceed.
Other suggestions on how to configure these groupings to improve this in future would be welcome.