Add helper object to access parameters of pixel diodes.

This allows to avoid consecutive pixel diode searches in case multiple diode parameters need to be accessed. This also allows to access the diode width in both directions.

Use this new access method to avoid consecutive diode searches to get the center position and width of pixel clusters.

This addresses ATLITKSW-303.

Vtune suggest that this speeds up the Acts pixel clusterization by 18%, Perfmon suggests a 3% overall improvement in a acts tracking only job over 100 events with 8 threads.