Create imbalance report

[elphick]

Imbalances across a network can involve all components or only some components, on some or all records in a dataset.
To complement the visual aspect of the existing plot_balance method we need a report that summarises the imbalances across the entire network.

The "bones" of this exist on the MCNode object - we just need to loop through all non-input/output nodes and call that, then concatenate, style, done.

[elphick]

Note to self - Some local commits have been made - still WIP.