History terminology misused in docs and output

[rtownson]

There are a number of places in the documentation and application outputs where the word 'histories' is used, but really should be 'number of source particles'. For a simple source, these two things are equal, but for a phase-space, shared library source, or radionuclide source they are likely different since the source particles include secondaries.

E.g. Fresh simulation of 10000 histories should be Fresh simulation of 10000 source particles.

We should also consider adding a feature to ask for a number of histories to simulate, instead of ncase which is the number of source particles to simulate.