Remove the `DistributionFamily` class.

[willGraham01]

Full context: #47

Remove the DistributionFamily class

Though we initially thought we would need it, in fact it seems we do not need (and will not need) the functionality the DistributionFamily (and subclasses) provide. We can instead just give DistributionNode._dist a Distribution class, and replace statements like Node._dist.construct(**) with Node._dist(**).

We should do this just to ensure the changes in the next stages don't have to straddle more classes than necessary.