Try the COMPSs @reduction decorator

[salvisolamartinell]

Check which algorithms could use the @reduction decorator from COMPSs, compare the performance, and incorporate it if it's better.

Off the top of my head: KMeans, Gaussian Mixtures, LinearRegression, PCA, and probably more could use it.

We should also check how to adapt the algorithms using an arity parameter.

[rsirvent]

Tried with magic method matmul (branch matmul_reduce), no gains on execution time, since previous code was already doing a reduce done by hand, and tasks are very well balanced.