Fix description of date_ranges

[jdossgollin]

This PR updates the documentation in the described calc_suite_specs argument to submit_mult_calcs in automate.py. Specifically, it fixes old documentation which called for a tuple of datetime.datetime objects, and clarifies that (1) a list of tuples is expected, and (2) other objects, which may be datetime.datetime but also other interpretable objects, are allowed.

closes: #295

[spencerkclark]

Great, thanks for following up on this @jdossgollin!

[spencerahill]

Great, thanks @jdossgollin!