WMTS Capabilities Content node does not allow layers to be added

[travis-val]

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:

<ows:DatasetDescriptionSummary xsi:type="LayerType">

Instead of:

<Layer>

A possible solution is this commit.