Fix Iris/Sigmet, align with CfRadial2

[kmuehlbauer]

This fixes the issue with #51 and removes some inconsistencies with the common CfRadial2-based API. Also undefaulted angle reindexing and removed a1gate and angle_res from angle-coordinate.

ping @aladinor @mgrover1

[codecov[bot]]

Codecov Report

Merging #53 (a425c30) into main (25f3c06) will decrease coverage by 0.86%. The diff coverage is 83.87%.

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
- Coverage   85.41%   84.55%   -0.87%     
==========================================
  Files          17       17              
  Lines        3119     3120       +1     
==========================================
- Hits         2664     2638      -26     
- Misses        455      482      +27     

Flag	Coverage Δ
unittests	84.55% <83.87%> (-0.87%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
xradar/io/backends/iris.py	79.11% <80.95%> (-2.51%)	:arrow_down:
xradar/model.py	94.28% <90.00%> (+0.03%)	:arrow_up:
xradar/io/backends/common.py	88.57% <0.00%> (-0.72%)	:arrow_down:

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[kmuehlbauer]

Approved assuming all is good with Vaisala :)

We can still change later, if it turns out other way.