Configurable maximum variance for ACTS space point formation

• adds properties to steer the space points Z/R variance
• we suspect that a smaller variance than the one computed can be used and using the cluster width may not be useful - this can be tuned systematically after this change
• I still see significantly more fake seeds at the end of the barrel at |\eta| \approx 1 than at |\eta| \approx 0 ... needs further investigation