Multi coverage by time plot

[EdwinB12]

Adds functionality to plot coverage by time steps.

Add to notebook by:

time_zones = [(i, i + 10) for i in range(0, 90, 10)]
plots.plot_coverage(preds, trues, time_zones=time_zones)

time_zones is None by default and therefore normal coverage plot is used.

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[EdwinB12]

To be merged on @sgreenbury coverage branch

[sgreenbury]

Looks great, thanks @EdwinB12! A few comments:

• Would it be possible to replace time_zones with windows in the API and docs etc since that's the API elsewhere?
• Would it help for MultiCoverageByTime to subclass MultiCoverage and reduce any repeated code?
• Perhaps we could add further class or extend API to support returning the coverage fully spatiotemporally too though I think we can look at this in a separate PR. Being addressed as part of #212.