Plot for sea-ice extent over forecast period / observational comparison improvements

[JimCircadian]

It'd be nice if we could plot the calculated sea ice extent (we can already plot the SIE error) as well as extending the existing absolute SIE comparison to allow for integrated ice edge error comparisons.

CC @ampersandmcd as I know you've investigated IIEE before.

This is a multi-faceted issue, so tick off / break down to new issues as required, but should be a relatively simple extension of the very extenstively developed icenet.plotting submodule: it'll be great for someone keep to get used to the existing codebase to familiarise themselves with developing icenet.