Use numpy arrays as default return value

[axsaucedo]

Using python list has a flaw given that by default python floats are C++ doubles, so there are situations where there is nuanced differences even when returning a rounded up float. To avoid these ambiguities it's best to ensure we always use numpy arrays as return values - similarly we currently also support numpy arrays as parameter so users shoudl eb aware that there may be some loss if this is not approached consciously.