Uniform prior is slow

Evaluating the Uniform.prob method is super slow considering what it's doing.

This is because we use the scipy method rather than the analytic version directly.

This is probably also an issue for other priors that use the scipy implementation.

We should:

• profile all of the priors which use scipy.
• see if we can make them run faster, sometimes the checking can be avoided by using the ._prob method rather than the .prob method.
• if not, just code in the prob and logprob methods ourself.