Implement dynamic result records

[jpswinski]

The current protocol used to communicate between the SlideRule server and the client is the internal RecordObject protocol which has a static record definition for each record type. This does not support newly added fields.

If the user wants certain atypical data returned as a result of their algorithm processing request (for instance say they want the associated surface type classification for each photon), then we need to have the ability for the user to specify what they want in their request and for the server to build a custom result record out of what is asked.

We can't include everything all the time because that would waste a lot of bandwidth. Also, if we had this feature, then for very large requests the user could only ask for a few things in the record which could greatly speed up the transfer of the results.

[jpswinski]

This has been implemented as a part of the ancillary records:

• there is an extent ancillary record which targets the geolocation and geophys_corr subgroups
• there is a photon ancillary record which targets the heights subgroup

Support for more subgroups will be forthcoming.

The main part of the implementation is the use of a unique extent_id that is generated for every extent. This id is then used in any ancillary records to identify which extent the ancillary data goes along with.