Nicole/exit codes - Githubissues

This PR:

Changes commons.sh to common_utils.sh and /sel735/commons.sh to common_sel735.sh
Checks for args before parsing args
Adds specific exit codes to failures and log statements
Added validate_directory_complete to check for data and metadata
Changes sourcing scripts in download.sh to executing them
Sources common_utils in child scripts so can be called/tested separately (maybe can do this in bats?)

acep-uaf / camio-meter-streams