Revise parsers for service object types

[roomthily]

In the v0 set of parser objects (technically "preprocessors", the actual parser is pretty generic for any xml), the response is pretty much geared towards the service-level metadata extraction. But with the ontology changes, we'll need dataset-level (at least) as well.

The response structure needs to be revised for that: service or dataset, remainders, and the BaseReader class revised to not assume service descriptors at least.

[roomthily]

See #25 for possible OWSLib integration for any OGC service (capabilities service metadata or dataset/layer metadata) handling.