`randomize_hyperparameters` ignores `prior_on`

The `randomize_hyperparameters` helper does not check parameters' `prior_on` attributes [here](https://github.com/secondmind-labs/trieste/blob/aa94c861c360c4ef4710eb94e837f598a81dcbae/trieste/models/gpflow/utils.py#L79). Hopefully a quick fix:
```
if param.prior_on is PriorOn.UNCONSTRAINED:
    param.unconstrained_variable.assign(sample)
else:
    param.assign(sample)
```