Burst2safe

Fixed
* Bug in CLI parsing of granule case for `burst2safe`.

Removed
* `--pols` as a required argument for `burst2safe` and `burst2stack`. Default value is now `VV`.

Added
* `burst2stack` tool for creating stacks of SAFEs.
* The ability to specify swaths and minimum number of bursts when using tool.
* The ability to specify the SAFE extent by either bounding box or vector file.

Fixed
* `Safe.get_name()` so that it correctly parses `Safe` objects with only cross-pol data.

Changed
* Moved all search/download functionality to `search.py` module.
* `--bbox` argument to `--extent`.

Fixed
* Polarization code now accurately reflects bursts contained in SAFE.
* Measurement GeoTiff metadata now correctly specifies Sentinel-1 A or B.

Added
* CLI argument for specifying output directory.

Added
* Separate check of Earthdata credentials prior to download.

Added
* Retries of download functionality to improve robustness of download step.

Fixed
* Bug introduced in `0.3.1` where the `download_bursts` function would not work for a single worker.