optimize import of Sentinel 2 data when possible

[Dimitar Misev]

In the slicer section we can add support for the following option:

• subtype - Specify a slicer subtype. Currently only "sentinel2" is supported as a value, valid in combination with "type": "gdal". When present in the ingredients, instead of opening files to be imported with GDAL in order to read needed metadata such as CRS and geo bbox, this will be done by reading the MTD_TL.xml file present in the SAFE container that contains the file to be imported. This can be much faster compared to reading the metadata from JPEG 2000 files with GDAL. Note that this only works with file paths on the filesystem in extracted SAFE directories.