Improvements to the core prior class

Cherry picked results from !905 (merged):

• Add ignore errstate to avoid warning messages
• Add helper properties for the width, non_fixed_keys, fixed_keys, and constraint_keys
• Add constraints to the ConditionalPrior

changed milestone to %1.1.2

added Priors label

added 3 commits

• 15f5d624...fef914d0 - 2 commits from branch master
• eb99323d - Merge branch 'master' into core-prior-improvements

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merged

mentioned in commit 4e3e6892