Reorder prior reading

This MR reorders the set of things attempted when trying to parse the prior argument strings. The previous method was causing issues, e.g., the one identified in !617 (merged).

I hope the new order makes a bit more logical sense than the old way. A more descriptive error message is raised and I added a docstring so that future developers will hopefully follow better what is going on.

This will strip the r'' from any parameters that are passed as raw strings, e.g., r'foo' will just render as foo.

@sylvia.biscoveanu can you test this on the prior you had issues with.