[WIP] Mapproxy service extension

The aim of this PR is to provide WMTS tiles for other _projection_ than the used by map.geo.admin.ch using _MapProxy. The ultimate goal would be to use the same RESTful patten for _TileMatrixSet_ like ETRS89, WG89, LV95 and the ubiquitous spherical mercator.

_http://wmts.geo.admin.ch/1.0.0/ch.swisstopo.pixelkarte-farbe/default/{Time}/{TileMatrixSet}/16/1/5.jpeg_

The config file for _MapProxy, `mapproxy.yaml` is generated by a script reading the layers list from the BOD.

1. Using a WMTS source is no issue for MapProxy, but we have to define a new _layer_ for each _projection_, for instance `pixelkarte`is the original name, `pixelkarte.mercator` would be the name for the reprojected layer in mercator, which is redondand.
2. It is now not possible to forward dimensions to sources other than WMS one. 
3. The Mapproxy demo page does not support dimension (nice to test, but irrelevant for the service itself)


If this part is executed during buildout, I would not commit mapproxy.yaml