popRates should be renamed to popInfo

[salvadord]

@joewgraham Since in the new plotRaster function, the 'popRates' argument shows not only rates, but also num cells and num syns, it should probably be renamed to 'popInfo'

Alternatively, can add popNumCells and popNumSyns as boolean arguments. This would actually keep backward compatibility. Right now, I have to add 'popRates="minimal"' to all the large scale models so that the legend fits.

https://github.com/Neurosim-lab/netpyne/blob/e6e6481125963fc6d1a209bc96980d3a9944fd43/netpyne/plotting/plotRaster.py#L156

[joewgraham]

I like the idea of switching to popInfo better... I like having fewer parameters and I don't like the idea of giving so many possible permutations of info to include.

We could change the default to "minimal", but for the smaller networks I've been dealing with, I like to see the additional pop info right on the plot... What do you think, @salvadord ?