Add gh actions

[rosesyrett]

PR to remove azure pipelines and replace them with GH actions, just because it's more transparent for other developers.

[codecov]

Codecov Report

Merging #49 (6ab4448) into main (4b37d5e) will increase coverage by 1.86%.
The diff coverage is 80.53%.

@@            Coverage Diff             @@
##             main      #49      +/-   ##
==========================================
+ Coverage   80.30%   82.17%   +1.86%     
==========================================
  Files           9        9              
  Lines        2280     2322      +42     
  Branches      391      372      -19     
==========================================
+ Hits         1831     1908      +77     
+ Misses        318      295      -23     
+ Partials      131      119      -12     

Impacted Files	Coverage Δ
src/diffcalc/ub/crystal.py	76.06% <56.25%> (+0.62%)	⬆️
src/diffcalc/ub/fitting.py	81.30% <66.66%> (ø)
src/diffcalc/hkl/calc.py	74.39% <76.27%> (+2.76%)	⬆️
src/diffcalc/ub/reference.py	93.47% <91.83%> (+2.74%)	⬆️
src/diffcalc/ub/calc.py	81.53% <94.28%> (+2.09%)	⬆️
src/diffcalc/hkl/constraints.py	93.63% <100.00%> (ø)
src/diffcalc/hkl/geometry.py	81.81% <100.00%> (ø)
src/diffcalc/util.py	100.00% <100.00%> (ø)

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