Add CentroidPoint class
Add computeCentroid() functions that use CentroidPoint
Extract centroid-related tests from test_common.cpp

There are quite a number of functions dealing with centroid, mean, and
covariance computation. They all are grouped in 'common/centroid.h'
header. Therefore it makes sense to group their corresponding test
functions in a separate file. Hence creation of 'test_centroid.cpp'.
Add unit tests for CentroidPoint and computeCentroid()
Merge pull request #586 from taketwo/add-centroid-point

Add a new `CentroidPoint` class