Map Imputing Zeroes: Support new variable annotations

[d-callan]

the new annotations are:

1. hasStudyDependentVocabulary w either true/false or yes/no values (@bobular would know i suppose) on variables (on the sample entity).
2. variableSpecToImputeZeroesFor is also on variables (on the sample entity) w values like 'entityId.variableId' which tell us since these data are tall where the zero value actually goes.

[d-callan]

also worth coordinating w @jbrestel, who was talking about changing the way the variable annotations are handled in the db iirc

[bobular]

There's also weightingVariableSpec - it doesn't seem to have a ticket but there is code to support it in veupathUtils