Large number of molecules overflows int in analysis modules

[trisyoungs]

Describe the bug
When one has a large number of molecules (approx 50k) and calculates any site-site correlation between them, if the normalisation depends on the square of the number of sites this overflows a standard 32-bit int (~2.4 billion) and wraps back round to negative numbers. This causes RDFs, for example, to become inverted about y.