Add initial sentinel1 accessor that raises when not used on backend Dataset (more or less)

[alexamici]

Objectives for the accessors are:

• [ ] exposes a hierarchical dataset tree where accessing group datasets without the need to open_dataset again #7
• [ ] exposes original product exploration functions, especially the one that may require slow access when on a network #15
• [ ] exposes a way to get as much as possible of the original metadata #7
• [ ] exposes API to write STAC catalogs to the product folder
• [x] raises a user-friendly error message when not used on a sentinel-1 Dataset

[codecov-commenter]

Codecov Report

Merging #46 (60eeaaf) into main (988a162) will increase coverage by 0.19%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
+ Coverage   93.12%   93.32%   +0.19%     
==========================================
  Files           7        7              
  Lines         538      554      +16     
  Branches       62       64       +2     
==========================================
+ Hits          501      517      +16     
  Misses         20       20              
  Partials       17       17              

Impacted Files	Coverage Δ
tests/test_xarray.py	100.00% <100.00%> (ø)
xarray_sentinel/sentinel1.py	95.02% <100.00%> (+0.20%)	:arrow_up:

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[aurghs]

this PR seems to be too big. I think we can split it into smaller ones. One for each point of the list.

[alexamici]

I agree.