Add Support For Custom span_range values

Need to create a function to grab all data points between time span ranges. It would be pretty cool to allow for span_ranges of 'minutes=90', or something of the sort.

Start by locating the existing span_range handling and the tests covering range-based data-point selection. Determine how current span values are parsed, define the expected behavior for values such as “minutes=90,” and add coverage showing that all points within the requested span are returned.

Written by the indexing model from the issue text.