Add probability option to aug nodes

[sheridana]

• Adds a skip_node() batch filter method to be overridden in subclasses
• Adds a probability parameter to augmentation nodes that returns true in skip_node() if less than a random float between 0 and 1
• Adds a test for skipping

Pre-Merge Checklist:

• if this PR adds a feature: request merge into the latest development branch (vX.Y-dev)
• if this PR fixes a bug: request merge into the latest patch branch (patch-X.Y.Z)
• PR branch name is short and describes the feature/bug addressed in this PR
• commit messages are consistent
• changes reviewed by another contributor
• tests cover changes
• all tests pass

[codecov-commenter]

Codecov Report

Merging #192 (d29cbb9) into dev-v1.4 (a33e5b4) will decrease coverage by 0.03%.
The diff coverage is 93.75%.

@@             Coverage Diff              @@
##           dev-v1.4     #192      +/-   ##
============================================
- Coverage     67.60%   67.58%   -0.03%     
============================================
  Files            98       98              
  Lines          6829     6854      +25     
============================================
+ Hits           4617     4632      +15     
- Misses         2212     2222      +10     

Files	Coverage Δ
gunpowder/nodes/batch_filter.py	91.08% <100.00%> (+0.18%)	⬆️
gunpowder/nodes/deform_augment.py	91.54% <100.00%> (+0.12%)	⬆️
gunpowder/nodes/elastic_augment.py	84.36% <100.00%> (+0.19%)	⬆️
gunpowder/nodes/intensity_augment.py	94.73% <100.00%> (+0.61%)	⬆️
gunpowder/nodes/noise_augment.py	100.00% <100.00%> (ø)
gunpowder/nodes/shift_augment.py	99.24% <100.00%> (+0.01%)	⬆️
gunpowder/nodes/simple_augment.py	93.58% <100.00%> (+0.12%)	⬆️
gunpowder/nodes/defect_augment.py	13.10% <33.33%> (+0.42%)	⬆️

... and 1 file with indirect coverage changes