Bug fixes in xval and report

[tamarl08]

• fix the concatenation of prediction files from xval folds, which used the keys param incorrectly and unnecessarily. This caused a bug only with more than 9 folds.
• added a 10-fold scenario to the test of that method
• fixed the description of human-system confusion matrix to match the matrix axes values.
• changed the title of human-human confusion matrix to be different than the human-system one, so that the links at the top works correctly.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Files	Coverage Δ
rsmtool/utils/cross_validation.py	95.34% <100.00%> (ø)

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