Support additional array-like containers

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Astropy contains several additional quantified array-like containers that provide utility for clarity and conversions, such as:

• Time
• TimeDelta
• Angle
• Longitude
• Latitude
• Distance

The challenge will be in deciding how to represent this metadata in attributes.

Some of these are listed in cf standard_name table, where a standard_name attribute could serve as a universal/structured way of hinting that parsing into specialised containers is suitable, especially logarithmic unit references.

Unfortunately, time is largely undocumented by CF, which only describes that it is canonically of units s, but the "air temperature" tutorial dataset assigns it UNIX datetime64 (presumably on UTC scale) instead of floating-point seconds. Astropy additionally does not use unit attributes for time containers, it instead splits units into format and scale.

Additionally, xradio msv4 uses the attribute type to represent a similar idea; however, it proposes several time types to represent inter-dataset relations of its schema and is possibly unfinalized.

To avoid conflicting with CF and xradio standards, it may be wiser to either:

• Use standard_name="datetime" and standard_name="datetimedelta"
• Use a key other standard_name for time
• Use entirely separate keys to CF and xradio, and instead require an attribute converter function to load foreign datasets with astropy containers.