MaterialInteractions methods do not need "this" so can become static

• MaterialInteraction methods can be made "static" because they don’t use this. e.g [https://clang.llvm.org/extra/clang-tidy/checks/readability-convert-member-functions-to-static.html]

• In future we can consider turning the struct to a namespace and have just function inside a namespace which is effectively the case since they are no data members/state etc.

• Update clients

• Simplify the calculation of the Ionization energy loss for the GSF fitter