Investigate supplementary GCC warnings (was ACTSFW-83)

Original author Hadrien Benjamin Grasland @hgraslan.

I just tried running a test framework build with maximal GCC warnings on (-Wall -Wextra -pedantic). While many of them were false positives (e.g. unused function parameters in virtual interfaces) or minor details (e.g. extraneous semicolons), there were a number of things that looked like potential bugs worth investigating.

For example, RootIndexedMaterialReader fetches data from a ROOT file... and then basically throws it away. That's probably not what one would expect it to do.