0.3.0

• Added optional geographical bounding box parameter bbox.
  Using a STAC API to filter source files accordingly.
  bbox is None by default, which means global coverage. (#20)
• Added optional parameter res_level in the range from 0 (default) to 4
  to let users choose their desired spatial resolution.
  A parameter spatial_res is not suitable,
  because resolutions are provided at fixed levels using the DGG. (#21)
• Added module smos_box.catalog.stac with new SmosStacCatalog
  that is now the default catalog used by the SMOS data store.
• Removed module smos_box.catalog.index including SmosIndexCatalog
  and dropped package smos_box.nckcindex entirely.
  We are no longer using a Kerchunk reference file index.

Full Changelog: v0.2.2...v0.3.0