This follows from !37993 (closed) which I closed as it picked up a lot of conflicts after urgent bug fixing.
Primarily improve error reporting so use of at() with an index out of range is trapped and reports a meaningful ATH_MSG_FATAL message instead of creating an exception (in context of ATLIDTRKCP-302). Other cleanups done: use of cmath constants instead of TMath, tidy up includes, use of std::array instead of a c style array, clarification of the declaration and usage of a pointer-to-member-function variable.
