53 improve orbit vector definition

[aurghs]

close #53 modified orbit dataset structure as suggested in #53, with some differences:

• axis instead of i
• x, y, z instead of 0, 1, 2 for axis names.

<xarray.Dataset>
Dimensions:       (azimuth_time: 17, i: 3)
Coordinates:
  * azimuth_time  (azimuth_time) datetime64[ns] 2020-03-15T17:04:38.537823 .....
  * axis          (axis) <U1 'x' 'y' 'z'
Data variables:
    position             (azimuth_time, i) float64 5.659e+06 5.618e+06 ... 4.926e+06
    velocity             (azimuth_time, i) float64 -4.072e+03 -4.138e+03 ... -5.078e+03
Attributes:
    reference_system:  Earth Fixed
    Conventions:       CF-1.7
    title:             Orbit information used by the IPF during processing
    comment:           The dataset contains a sets of orbit state vectors tha...
    history:           created by xarray_sentinel-0.1.2.dev55+gb940721.d20210508

• Updated tests and README.md

[codecov-commenter]

Codecov Report

Merging #55 (66112fc) into main (b940721) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
+ Coverage   92.64%   92.67%   +0.02%     
==========================================
  Files           7        7              
  Lines         612      614       +2     
  Branches       68       68              
==========================================
+ Hits          567      569       +2     
  Misses         24       24              
  Partials       21       21              

Impacted Files	Coverage Δ
tests/test_sentinel1.py	100.00% <100.00%> (ø)
tests/test_xarray.py	100.00% <100.00%> (ø)
xarray_sentinel/sentinel1.py	92.53% <100.00%> (+0.06%)	:arrow_up:

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