create_cov: use updated COSMOMC keys

[RickKessler]

9.22.2021 RK - check legacy keys

key_legacy_list = [ ‘COSMOMC_TEMPLATES’, ‘DATASET_FILE’,
‘SYSFILE’ ]
key_update_list = [ ‘COSMOMC_TEMPLATES_PATH’, ‘COSMOMC_DATASET_FILE’,
‘SYS_SCALE_FILE’ ]

in short, please use keys from key_update_list. Would also be useful to have an option to exclude the /cosmomc directory to reduce the large output for unbinned COV matrices.