Eliminate independently-defined LogNormalScatter class

Defining scatter in an independent class is a bit convoluted because the functionality is so simple. After some experimenting with building models, reliance on this class does not really improve convenience. It is probably better to entirely do away with this class and just have any stellar-to-halo-mass relation implement its own scatter. The downside is a mild amount of code duplication will be required, but I think this outweighed by the improved clarity and relaxed restrictions on future development.

It may be better to similarly refactor the stellar-to-halo-mass relations so that PrimGalPropModel is also entirely eliminated. @johannesulf or @duncandc - do either of you have an opinion on this? If it makes no difference to either of you, I'll probably just go ahead and eliminate both of these classes.