Support WMTS as base layer

[felix-mu]

Add WMTS as map base layer type:
https://openlayers.org/en/latest/examples/wmts.html

[felix-mu]

https://openlayers.org/en/latest/examples/wmts-layer-from-capabilities.html

[felix-mu]

https://openlayers.org/en/latest/apidoc/module-ol_format_WMTSCapabilities-WMTSCapabilities.html