Variance flooring is not set properly in the constructor

For some reason the variance threshold is not set in the initialization of the GMMTrainers (ML and MAP).

Currently, the default is set to 0.0 and not to: either the minimum limit value (if nothing is set) or the one explicitly set in the constructor. The only case that this works is when you set this value explicitly via the method set_variance_thresholds (e.g http://pythonhosted.org/bob.learn.em/py_api.html#bob.learn.em.GMMMachine.set_variance_thresholds).