ConditionalPriorDict: fix subset sampling for external 'DeltaFunction' dependencies

Import patch and tests from https://github.com/lscsoft/bilby/issues/17. Credit to Reinhard Prix

assigned to @colm.talbot

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• 327f221f...0b57cbe8 - 12 commits from branch master
• 2c08bfe2 - Import patch and tests from https://github.com/lscsoft/bilby/issues/17

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merged

changed milestone to %2.2.1

mentioned in merge request !1305 (merged)