feat/get_oldest_file's_timestamp

@haufs @karnem a modification to get_dir_creation_date() function in cal_tools for returning the timestamp of the oldest .h5 file in the run directory. That is instead of returning the timestamp for the whole directory.

LGTM

added 1 commit

• 2e5827e9 - edit func. descr.

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merged

mentioned in commit 606d1287