Add ascending node time

[aurghs]

More work on #36.

[alexamici]

@aurghs unit test fails due to README using burst_index, you may run make doc-test to test the README.

Please run black, you may run make to fix code formatting and run all tests.

[codecov-commenter]

Codecov Report

Merging #68 (46fc55f) into main (aff094a) will decrease coverage by 2.11%. The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main      #68      +/-   ##
==========================================
- Coverage   95.67%   93.56%   -2.12%     
==========================================
  Files          12       12              
  Lines         694      715      +21     
  Branches       71       75       +4     
==========================================
+ Hits          664      669       +5     
- Misses         16       30      +14     
- Partials       14       16       +2     

Impacted Files	Coverage Δ
xarray_sentinel/sentinel1.py	90.97% <33.33%> (-5.29%)	:arrow_down:

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[alexamici]

@aurghs very nice!

[alexamici]

@aurghs one more thing: in clip_burst_dataset it would be nice to add an attribute with the azimuth_anx_time of the burst. Not sure what is best whether the first line of the central line, or maybe both.