Scikit-validate

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- Disabled log scale for diff plot. [kreczko]
- Setting minY to non-zero for logarithmic plots. [kreczko]
- Disabled logY setting if negative values are found. [kreczko]

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- Execute_with_metrics: added soft-fail to IOException on writing
metrics file. [kreczko]
- Added tests for get_target_branch. [kreczko]
- Fix missing import. [kreczko]

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- Improved error-handling for get_target_branch and added target &
default branches. [kreczko]
- Worked through the stricter pep8 set. [Lukasz Kreczko]
- Added default target_branch to get_target_branch. [Lukasz Kreczko]

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- Visualisation adjustments for root_diff. [kreczko]
- Fixed python3 issues with io._walk. [kreczko]

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- Replaced io._walk with a more robust equivalent. [kreczko]
- Added automated logy & x-limits to drawing. [kreczko]
- Added code to generate tests/samples/objects.root. [kreczko]
- Steeled diff calculation, fixed normalisation (now w.r.t. reference)
and reporting non-comparible branches in root_diff. [kreczko]
- Made io.unpack more robust to str-arrays. [kreczko]
- Fixed pep8 error: unused include in test_io. [kreczko]
- Fixed "make test" [kreczko]

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- Added comparison of object data to tests. [kreczko]
- Updated io.walk to handle & unpack objects. [kreczko]