Cogeo-mosaic

* Raise Exception when trying to overwrite a mosaic (112)
* Add `reverse` option in `.tile` and `.point` to get values from assets in reversed order.

* Allow PointOutsideBounds exception for `point` method (108)

* BaseBackend.center returns value from the mosaic definition (105)

* BaseBackend is now a subclass of rio-tiler.io.base.BaseReader (add minzoom, maxzoom, bounds properties and info method)
* use `attr` to define backend classes

Breaking changes
* `backend_options` is now used to pass options (*kwargs) to the `_read` method

* update to rio-tiler 2.0b5

Breaking changes
* 'value' -> 'values' in MosaicBackend.point output (98)

* Use environement variable to set/disable cache (93, autho geospatial-jeff)
* Allow Threads configuration for overview command (author kylebarron)
* add --in-memory/--no-in-memory to control temporary files creation for `overview` function.
* allow pixel_selection method options for `overview` function.
* update to rio-tiler 2.0b4
* use new COGReader and STACReader to add .tile and .point methods directly in the backends

Breaking changes
* backend.tile -> backend.assets_for_tile
* backend.point -> backend.assets_for_point