Constant memory allocation for PicardTools

Independent of cluster.json configurations for each rule, PicardTools using constant memory for execution (20Gb).

# 20G for all Picard commands picard_command = "picard -Xmx20g "

This can cause problems in execution of large files ( >6gb after alignment) particularly for FixmateInformation rule. Is it possible to make memory parameter linked to the cluster.json parameters?