Dirichlet Prior: Fix reading prior that has been written to file

@salvatore-vitale pointed out that there is a bug that makes the Dirichlet prior objects unreadable due to a name mismatch.

This MR standardizes the name and adds some testing of the Dirichlet class.

added Bug Priors labels

assigned to @colm.talbot

added 1 commit

• fe460330 - Add missing imports

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mentioned in commit c9a86cbb