Better handling of data with an irregular time axis

[znichollscr]

The motivation

The volcanic data doesn't have a regular time axis, data appears sporadically. We don't handle this well at the moment, for example the time range on the written output data isn't as we'd like.

The proposed solution

• once we've worked out what we need to support, update the package to handle it
• write some docs to help with this

Alternatives

Additional context

This arises in the context of the volcanic emissions for input4MIPs, but may appear elsewhere too.