gh-9 Skip empty strings '' for enumerations. Save summary metadata to…

[aaronforshaw]

… data class as well as data element

Closes #9

1. When counting or selecting distinct enumeration values, append WHERE {column} <> ''to the queries, to prevent empty values being converted into null enumerations. This does not affect summary metadata, which still includes counts for empty strings.
2. Save summary metadata to the data class as well as the data element
3. Use the LongIntervalHelper when dealing with integers. When you have a large negative minimum (Integer) and a large positive maximum (Integer), the difference between them can be Long.