Add get_ranges query function

[angus-g]

This function returns a DataFrame containing the distinct ranges of contiguous data. For example,

>>> cc.querying.get_ranges(session, "01deg_jra55v13_ryf9091", "u", "1 daily")
                 start                  end
0  1950-01-01 00:00:00  1950-02-01 00:00:00
1  1950-02-01 00:00:00  1971-01-01 00:00:00
2  2086-01-01 00:00:00  2100-10-01 00:00:00
3  2100-10-01 00:00:00  2101-01-01 00:00:00
4  2170-01-01 00:00:00  2180-01-01 00:00:00

Curiously, this example does highlight an inconsistency in the data for this experiment, but it does show the gaps where no daily velocity data is available.

Probably not quite ready (and definitely needs some tests!), but I thought I'd get this out there while it's fresh on my mind.