Improve color_by_time

[wx4stg]

This allows for coloring numpy datetime64[ns] (used by event_time and flash_time_start) and even regular floats. tlim is now an optional kwarg, if it is not provided, it is assumed that the min and max of the time_array are the upper and lower bounds for tlim

[codecov-commenter]

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[deeplycloudy]

This moves us away from the python datetime API, and toward numpy's datetime64, which I like as a time format to which we promote all datasets. Tests, etc. LGTM. Thanks for not breaking earlier compatibility.