Have prior.size arg support multiple sizes, and remove prior.sizes

[eelcovdw]

When prior.sizes is defined, prior.size is not needed (since sum(prior.sizes) == prior.size).

Make prior.size behave the same way as prior.params; Multiple semi-colon separated values, which gets converted to a list of ints by the argument parser. Changes need to be applied in the arg parser, and in the code base wherever hparams.prior.size / hparams.prior.sizes is used.