We need to add methods to access datasets with datetime objects:

• filter_time_nearest returns a single dataset with the time closest to that requested
• filter_time_range returns a list of datasets falling inclusively into the time range. Start time is the first positional argument. The end time is keyword and defaults to utcnow.

Use a series of regular expressions to parse the filename string.

These leave us open to future expansion for where we want to source the times from (file name, time dimension, etc.)