Fix in-place modification of PrPixelTracking property

Don't update m_maxScatter in-place. This makes running Moore from a TCK that contains PrPixelTracking give consistent results to running from settings.

Without this fix then running from TCK gave different results, as the original parameter value was squared twice when running from a TCK.

cc: @rmatev @chasse @hschindl

See also: gaudi/Gaudi#16