update metadata

• make all parameteres consistently singular
• rename settings to config
• rename env_type to var_type
• rename environment (in input/output, settings) to environment_var
• rename inputs to input and outputs to output; and enable single value OR list
• add optional to input/output
• add [<SERVICE_DEFINITION>] to chain so that it can define its own compose service
• add <FURTHER_COMPOSE_DEFINITION> to chain and code velds