Dist-s1

Added
1. Minimum working example of enumeration of DIST-S1 products from MGRS tiles and (optionally) a track number in said MGRS tiles.

Changed
API is *very* much in flux to support enumeration of large spatial areas and to support the localization of RTC-S1 data. Will likely continue to change to promote changing.

The initial release of this library. This library provides:

1. Enumeration of DIST-S1-ALERT products. A DIST-S1-ALERT product can be uniquely identified (assuming a pre-image selection process is fixed)by:
+ MGRS tile
+ Acquisition time of the post-image
+ Track of the post-image
2. Ability to localize OPERA RTC-S1 data for the creation of the DIST-S1 product.