Allow 'total' (or 'All') as a population name?

[RomeshA]

'Total' could sometimes be a useful population name to have - same with 'all'. The reason these are not permitted is because they are used by PlotData as aggregations. 'All' is shorthand for selecting all populations, 'Total' is shorthand for summing all populations. However, currently None is treated the same as 'all' for this purpose. If 'all' was instead used as a shorthand for summing the populations, the keyword 'total' would be freed up. Alternatively, we could use 'total' instead of 'all', but in #490 it would be clearer if a single row entry in the databook was 'all' rather than 'total' because it's clearer how this relates to the values used in each individual population.

This would be a breaking change so would be slated for a major release

[RomeshA]

#490 means that 'All' cannot be used for this purpose any more, but 'total' could still potentially work. It might not always be clear to users though how the 'total' value is used though (considering that 'all' does now have implications for the simulations as the values are inserted as defaults - would users expect that a 'total' value is somehow disaggregated over populations? Or just that it's used for plotting?)