TimeSeries requires time quantity as datetime instead of float

[aalbino2]

Sometimes this would be the case:

    time = Quantity(
        type=datetime,
        description='The process time when each of the values were recorded.',
        shape=['*'],
    )

Instead of this:

    time = Quantity(
        type=float,
        unit='s',
        description='The process time when each of the values were recorded.',
        shape=['*'],
    )

We will need to discuss whether to support this more structurally, for now I'm simply subclassing and changing it case-by-case
@hampusnasstrom