Overriding __setattr__ to avoid having to explicitly call mean_subtract

[mwilensky768]

Currently the user must explicitly call ins.mean_subtract (AND explicitly make an assignment to ins.metric_ms). Since the metric_ms attribute is always given by the line ins.metric_ms = ins.mean_subtract(), this line is basically superfluous in many applications. This could be circumvented by overriding setattr() and adding an if clause to check if the metric_array is being changed. To fully generalize this, we may have to override numpy.ndarray.setitem

[mwilensky768]

This might be a better way to do linked attributes. Performance may suffer if having to recalculate the metric_ms many extra times. Will want to program around this.