(hopefully)

As this is consistent with the name used for Reflectometry reduction at ISIS.

The performance of the new code is not worth and even better (multithreading part) wrt the old code.

Simplified code by using a threadpool with max threads=1 per script
object. This leaves all the complex locking management to the
Qt framework code.

The Python property is actually a vector<long> so an extra template
instantiation was required for the toValue function.

Note that there are some minor modifications to the accessibility of some of the member functions on the Algorithm class. This is to facilitate reuse with the MultiPeriodGroupAlgorithm sub-class, which requires similar functionality.