Defines the interfaces to support the reading of DAS datasources by Apollo. The goal is allow different implementations to be provided of, for example, `DASServer`. These implementations can be sophisticated or not, as necessary.

I have included a reference (very direct, unsophisticated) implementation in the `apollo.dataadapter.das.simple` package.

The interface `DASServer` is the basic type referred to by a data adapter referencing a DAS datasource. The `DASDsn`, `DASSegment`, `DASFeature` and `DASSequence` are types used to pass information into a request from the server, and to retrieve information back from the server.