Extension of mapproxy to support new projections (WMTS)

All available layers and timestamps as physical tiles in S3,
but not use of keyword 'current'

- improving documentation and README for mapproxy
- removing OSM layers from config
- coping with empty parts in template
- adding missing projections in WMS
- optimized apache settings
- only requesting the needed WMTS tiles

For full doc, see https://github.com/geoadmin/mf-chsdi3/pull/865
If the topic is different from `api` and epsg is different from `21781` I would raise an error since some layers might just to be available. (related to your script)