WMTS Capabilities Content node does not allow layers to be added

The WMTS 1.0 ContentsType class is missing the ability to add a LayerType class. It can be added through one of the DatasetDescriptionSummary related methods, but this results in errors when the capabilities object is serialized. It will produce nodes named:

Instead of:

A possible solution is [this commit](https://github.com/travis-val/ogc-schemas/commit/0d3aaaf3a1a5c70800b8b9b2a6b7a4ac52c95759).

Start with the WMTS 1.0 ContentsType class and its DatasetDescriptionSummary-related methods, then reproduce serialization of a capabilities object containing a layer. Compare the behavior with the referenced commit and verify that the completed change serializes the layer as a Layer node rather than a DatasetDescriptionSummary node.

Written by the indexing model from the issue text.