adding ability to set DEFAULT_RUFFUS_CHECKSUM_LEVEL in environment

Modified the way checksum_level is set so that the DEFAULT_RUFFUS_CHECKSUM_LEVEL environment variable is checked if checksum_level is not set in the pipeline_* function params.
Order of preference:
pipeline_* param > env var > default (CHECKSUM_HISTORY_TIMESTAMPS)

There is one bit that I'm not sure about: the class _task definition in task.py sets checksum_level = CHECKSUM_FILE_TIMESTAMPS
I've left this, but I'm not sure if it will cause any issues.