[API][DATASTORE] Add /cube endpoint

[jo-asplin-met-no]

A /cube endpoint should be added to the API so that data within a bounding box can be retrieved (by specifying a bbox query parameter). The only difference from the /area endpoint will then be that the latter allows for retrieval within a spherical polygon (through the coords query parameter). Bounding boxes and spherical polygons serve different use cases, and both should be offered. In particular, spherical polygons allow arbitrarily many edges, but all of them follow great circle arcs, which is particularly significant for long edges along a mostly east-west direction.

Note that this requires datastore.proto to be extended with a BoundingBox bbox in the spatial filter section in the GetObsRequest message.