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").
Parameter description should contain the name of the parameter, i.e. in this case the line should be:

    \param[out] cluster_axes vector mapping each centroid to the number of signatures