Generic ATL03/06/08 subsetter to support cross-referencing calculated fields

[jpswinski]

Need to have an endpoint(s) that allows a user to request fields from ATL03/06/08 datasets. SlideRule then reads and subsets them, and then returns the values back to the user with a corresponding segment ID index for each value.

For example, if a user wanted to see all photon heights in a region, SlideRule would return a set of records where each record had the segment ID as the index value, the name of the field as a string, and a bunch of heights.

[jpswinski]

By applying a consistent segment ID to the calculated elevations as well as any subsetting fields, users can cross reference elevations with a large number of different fields in the ATL03/06/08 datasets.

[jpswinski]

The code uses a unique extent_id field per atl03 extent that is then used for cross-referencing other ancillary fields.