transformer_from_metric should be a helper function

[bellet]

Description

The function transformer_from_metric(metric) which decomposes a PSD matrix into a transformer is currently a method of the class MahalanobisMixin. I think the purpose of this function is to do the conversion at the end of the fit algorithm when the latter learns the metric as a PSD matrix instead of a linear transformation. Hence I think it should simply be a helper function.

[wdevazelhes]

Yes, I agree, indeed we shouldn't need to have a MahalanobisMixin object to be able to use it