Create a list of runs that we can traverse

Create a list of runs that we can traverse

create_run_list(runs, psm_dir = ".", check_psm_files = FALSE)

Arguments

runs	A list of lists. Each inner lists has an element named 'psm' that holds the psm string, and elements named using three letter crop acronyms, as used in FOCUS_PELMO_crops, that hold character vectors of three letter scenario acronyms as used in FOCUS_GW_scenarios_2012.
psm_dir	The directory where the psm files are located
check_psm_files	Should we check if the psm file exists