Rio-tiler

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- add `rio_tiler.mosaic` submodule (ref: https://github.com/cogeotiff/rio-tiler-mosaic/issues/16)

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- add boto3 in the dependency (201)

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- switch to ContextManager for COG and STAC (rio_cogeo.io.COGReader, rio_cogeo.io.STACReader).
- COGReader.part and STACReader.part return data in BBOX CRS by default.
- STACReader now accept URL (https, s3).
- add more method for STAC (prewiew/point/part/info/stats).
- add expression for COG/STAC preview/point/part.
- add `masked` option in `rio_tiler.reader.point` to control weither or not it should return None or a value.
- remove mission specific tilers (195).
- remove `rio_tiler.reader.multi_*` functions (replaced by rio_tiler.io.cogeo.multi_*).
- remove `rio_tiler.utils.expression` (replaced by expression options in tilers).

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- refactor `rio_tiler.utils.tile_exists` to allow raster bounds latitude == -90,90

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- Change default resampling to nearest for `_read` (187)
- add `rio_tiler.reader.stats` (return only array statistics)
- remove default `dst_crs` in `rio_tiler.reader.part` to to fallback to dataset CRS.

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- Refactor colormap and add method to register custom colormap