Update tracking for MDC

Fix problems which occurred in MDC reconstruction r0007:

• Create reference surface 1mm in front of the first measurement. This fixes the problem that tracks only have 17 hits.
• Set charge of negative particles to -1, not 0. This fixes the problem that all tracks have charge 1.
• Remove seeds with less than 10 hits or more than 6 shared hits. This fixes the problem with too large memory usage. It is particularly important for tracks which hit two overlapping SCT modules and have therefore 12 hits per station.