Let date be a date, not just datetime; also default _at() methods to now()

[jvavrek]

Closes #226

[cosama]

Looks good to me. Might make sense to add a test for the datetime.date thing.

Also there is this test: https://github.com/lbl-anp/becquerel/blob/update-date/tests/isotope_qty_test.py#L249. We could add no argument to it.

Looking at that I realized that there is a *_now method as well. Maybe can deprecate that and simplify the interface by defaulting to now with the *_at methods. @markbandstra Any thoughts?

[jvavrek]

@cosama this should be good to go:

• added a test for the datetime parse
• added no argument to test_isotopequantity_activity_now()
• deprecated the _now() methods

[cosama]

just realize there is only ref_atoms but no ref_bq or other units. Probably worth adding that? Here or new one? Maybe a ref_quantity as well?

[markbandstra]

Thanks!

[jvavrek]

just realize there is only ref_atoms but no ref_bq or other units. Probably worth adding that? Here or new one? Maybe a ref_quantity as well?

The ref_atoms docstring says:

Access the reference atoms directly (for backwards compatibility)

so I assume that self._ref_quantities is the preferred way.