Change BMS to take in priors and ops together

[TheLemonPig]

I realised that because BMS requires both and that they must match. If we allow the user to input their own priors then we need to make sure there is a matching ops dictionary. The simplest way I can see this is to require the user to input both themselves

[TheLemonPig]

Having worked with inputting custom functions, I now believe this change isn't necessary. This is because whenever the user wishes to add their own function to the primitives or to the priors, they would be required to point towards a callable function. Callable functions have the number of arguments as a built-in attribute so we wouldn't need to specify this.