Allow open-ended time ranges

[GypsyBojangles]

When specifying a time range, be able to have the start or end of the range be open, to indicate all data before or after a date. For example:

time=('2001', None)
could indicate all data from 2001 onward.

time=(None, '2010')
could indicate all data before 2010.

[stale]

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