Add helper method for getting proposition condition

[korikuzma]

Feature description

In propositions that have conditions, we should add a helper method to get the condition

Use case

• objectCondition, objectTumorType, and conditionQualifier are all condition field names on Propositions. I want to be able to call Proposition.get_condition() without having to know the name of the field

Proposed solution

No response

Alternatives considered

No response

Implementation details

No response

Potential Impact

No response

Additional context

No response

Contribution

Yes, I can create a PR for this feature.

[jsstevenson]

@korikuzma a setter method might be nice too