fix: fix TypeError when using Nyquist interp

[Tieqiong]

@sbillinge please check, thanks

Previously when using nyquest grid it will give error like

TypeError: '<' not supported between instances of 'list' and 'float'

which is because the sampler function in diffpy.utils only accept ndarray while pdfgui provide array_like (list) type

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.10%. Comparing base (9a0cf41) to head (8b3c51f).
Report is 3 commits behind head on main.

Additional details and impacted files

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  Coverage   95.10%   95.10%           
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  Files          23       23           
  Lines        1123     1123           
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  Hits         1068     1068           
  Misses         55       55