add com/dpc imaging modes to show4dstem

[henryhng]

• Add 6 center-of-mass virtual imaging modes that compute beam deflection maps from 4D-STEM dat:
  • com-x/com-y
  • com magnitude
  • iCOM - integrated COM via Fourier-space Poisson solver, recovers projected electrostatic potential
  • dCOM - divergence of COM field
  • curl (EM)
• Corresponding tests
• Rewrite show4dstem all_features notebook data generator to simulate SrTiO3 perovskite crystal (atomic potential gradient & BF disk shift, Bragg spots, thickness modulation) to achieve clearer/more obvious visualisations

[henryhng]

in test widget showcomplex, change NamedTemporaryFile to pytest's tmp_path fixture which gives an unlocked directory path (on Windows NamedTemporaryFile holds file locked so PIL cannot write to it)