Use pytorch for 3D+ data

[soupault]

Proof of concept.

• Switched to pytorch for 3D+ in Flip, Crop and Pad
• Flip now accepts multiple axes, but not axis=-1. Also, the axis order has been switched from opencv to numpy
• Minor cleanup to the tests

[codecov[bot]]

Codecov Report

Merging #71 into master will decrease coverage by 0.56%. The diff coverage is 86.20%.

@@             Coverage Diff             @@
##            master      #71      +/-   ##
===========================================
- Coverage   100.00%   99.43%   -0.57%     
===========================================
  Files           10       10              
  Lines         1388     1417      +29     
===========================================
+ Hits          1388     1409      +21     
- Misses           0        8       +8     

Impacted Files	Coverage Δ
solt/core/_base_transforms.py	98.26% <73.33%> (-1.74%)	:arrow_down:
solt/transforms/_transforms.py	99.43% <89.74%> (-0.57%)	:arrow_down:
solt/core/_data.py	100.00% <100.00%> (ø)

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[soupault]

Open points: cv2 2D, pt 3D, backend, device, gamma 3D, normalization