ITensor: Compiler speedup, restored QN Spectrum information, bug fixes

Reorganized library code for big speedup of driver-code compile times.

Restored computation of quantum numbers of entanglement spectrum. It is now only done if requested by passing the (boolean) named arg "ComputeQNs".

Fixed bug where AutoMPO was not working for non-QN conserving operators (thanks Yifan Tian).

Fixed bug where Truncate=false was getting set back to true if other truncation args (Cutoff, Maxm) were set (thanks Bill Huggins).

Fixed autovector to allow types with non-constexpr default constructor.

Changed averageM(MPS) to return a Real instead of an int.

Broke up some big source files to aid compilation on Windows using cygwin.

Updates to tutorials for Sherbrooke summer school