CRAN version

This release is a refactoring of the package, functions has been exploded to be
better tested and inspected.

fix

• update tests with valid provider
• remove (not functioning) Esri Delorme provider
• better management of cache, tiles and composed raster are cached
  (huge speed gain)
• remove the gdal-based merging mechanism, not efficient anymore.

feat

• improve doc in create_provider() for wms inspection (#22 @paul-carteron)
• add *.webp tile extension support (#28 @tadhg-moore)
• change the output of verbose
• cache composed raster for future use
• API keys management for Stadia and Thunderforest with env variable