Added a continuousRangeFilter that can support many-valued or non-discrete-valued attributes.

Filter for data with a continuous range or many possible values, such as dates, floats, etc.
Query with a scalar to query by exact value, or query with a length-2 array to
query a range (as in dvrangeFilter) such that the value is greater than or equal
to range[0] and less than range[1].

Closes #26.