Add a type that allows fetching data for each member of an input cohort (Reporting module)

/** RENAME THIS!
 * Fetches a single data item for each member of a cohort
 */
interface DataFetcher extends Definition {
    /** returns a map from each member id in the cohort to the fetched data item */
    public Map<Integer, Object> fetchData(Cohort cohort)
}

/** Always has an 'indexDate' parameter, in addition to whatever parameters come from the parsed logic expression */
class LogicDataFetcher implements DataFetcher {
    private String logicExpression;
     ...
}