Feature/test pandas spc x calc

[JPonsa]

unittest for nhspy_plotthedots.pandas_spc_calculations.pandas_spc_x_calc

I tried to reproduce the example shown in
https://www.england.nhs.uk/statistical-process-control-tool/
More tests would need to be included but further knowledge on the SPC would be required.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f2e3818) 19.56% compared to head (9120b07) 45.65%.
Report is 28 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #15       +/-   ##
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+ Coverage   19.56%   45.65%   +26.08%     
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  Files           8        8               
  Lines         138      138               
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+ Hits           27       63       +36     
+ Misses        111       75       -36     

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