Add a new `CentroidPoint` class

Originally a part of #473, this turned out to be a self-contained feature which might be useful not only inside `octree` module.

> A generic class that computes the centroid of points fed to it.
>
> Here by "centroid" we denote not just the mean of 3D point coordinates, but also mean of values in the other data fields. The general-purpose `computeNDCentroid()` function also implements this sort of functionality, however it does it in a "dumb" way, i.e. regardless of the semantics of the data inside a field it simply averages the values. In certain cases (e.g. for `x`, `y`, `z`, `intensity` fields) this behavior is reasonable, however in other cases (e.g. `rgb`, `rgba`, `label` fields) this does not lead to meaningful results.
>
> This class is capable of computing the centroid in a "smart" way, i.e. taking into account the meaning of the data inside fields. Currently the  following fields are supported:
>
 - XYZ (`x`, `y`, `z`)
>
>   Separate average for each field.
>
 - Normal (`normal_x`, `normal_y`, `normal_z`)
>
>   Separate average for each field, and the resulting vector is normalized.
>
 - Curvature (`curvature`)
>
>   Average.
>
 - RGB/RGBA (`rgb` or `rgba`)
>
>   Separate average for R, G, B, and alpha channels.
>
 - Intensity (`intensity`)
>
>   Average.
>
 - Label (`label`)
>
>   Majority vote.
>
> The template parameter defines the type of points that may be accumulated with this class. This may be an arbitrary PCL point type, and centroid computation will happen only for the fields that are present in it and are supported.
>
> Current centroid may be retrieved at any time using `get()`. Note that the function is templated on point type, so it is possible to fetch the centroid into a point type that differs from the type of points that are being accumulated. All the "extra" fields for which the centroid is not being calculated will be left untouched.

The class is accompanied by a couple of `computeCentroid()` functions (designed upon the existing `compute3DCentroid()` and `computeNDCentroid()`), plus a bunch of unit tests.