This will be used to fix the `GeneratorAdapter` when applied
to generators producing int-valued data.

Previously, the `GeneratorAdapter` applied `floatX` to float data,
but kept the original integer dtypes.
`floatX` was then applied to everything by `convert_observed_data`.

This refactor changes the handling of integer-valued generator data,
such that `intX` is applied, and no `floatX` conversion takes place.