imageToVTK: determine low-dim grid from spacing

[MuellerSeb]

Closes #37

imageToVTK was not able to determine lower dimension grids from given cell-data. Meaning: if cell data had a single slice in one direction, imageToVTK assumed it always to be 3D. But from given spacing of 0.0 one could determine that the cell elements should be 2D (or even 1D).

This PR fixes this issue and adds checks for spacing and data shapes.

[codecov-commenter]

Codecov Report

Merging #36 (a7e30e6) into master (f4df805) will decrease coverage by 1.34%. The diff coverage is 25.00%.

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@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
- Coverage   73.32%   71.99%   -1.34%     
==========================================
  Files           6        6              
  Lines         956      964       +8     
==========================================
- Hits          701      694       -7     
- Misses        255      270      +15     

Impacted Files	Coverage Δ
pyevtk/vtk.py	86.07% <ø> (ø)
pyevtk/hl.py	79.43% <25.00%> (-1.42%)	:arrow_down:

... and 1 file with indirect coverage changes

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[renefritze]

Thanks for the fix @MuellerSeb ! I've updated branch protection accordingly.

[MuellerSeb]

Thanks for the merge! Could you do a new release with that fix included? Need it downstream.

Cheers, Sebastian