Improvements to OUR-CVFH Estimation, including mapping from clusters to features and doc fixes

Fixes #737 by adding a `vector<short> cluster_axes_` to `OURCVFHEstimation`. This protected field consists of a count of the number of signatures that have been computed per cluster. It is set by `computeRFAndShapeDistribution` at the same time as `compute`'s output. When all axes are properly disambiguated, it will consist only of the value 1 (one entry per cluster). When some cluster i has multiple SGURF solutions, instead of 1, the vector will contain the number of SGURFs in location i.

Also fixes a typo in the documentation and another in the warning (plural of axis is axes, so "both axes" not "both axis").
Good catch. Just fixed it.