Created AMSimulationIO subpackage to handle reading/writing ROOT files
Updates following changes in fas package

Added lean_counter3 class
Updated lean_merger3 class
Added cust_useZeroMaterialTrackerOnly() in customise_pgun.py
Increased ranges in roads-related plots
Merge pull request #19 from jiafulow/prepare-v024

Created AMSimulationIO subpackage to handle reading/writing ROOT files