WIP: Prior clean up

Requires !688 (merged)

Some code improvements for the prior package:

• Cleaned up a bunch of warnings
• removed if/else-clauses for prob/rescale/ln_prob/cdf methods that distinguish between float and array input. It now always uses the array logic and then checks the type to return a float if it is a float input
• Introduced @valid_rescale_check and @consistent_type_use decorators to reduce redundant code throughout the package
• Restructured the prior tests into its own package
• Assured that prior saving/loading is still working the same way.
• Got rid off the check_range_nonzero argument in favour of dynamic testing with properties