Generalize population and connection indexing/labeling

[pausz]

From @RomeshA

There should be a way to name populations and give them a single
letter subscript, which would then enable us to refer to populations
with labels like 'e','r','s', and to connections like 'ee','ei'...
rather than population 1, couple 3. This would also make it easier to
compare similar components when the model is changing and thus the
indexes might change.

I could imagine a solution using something like Python dictionaries to have key:value pairs, where the key would be 'ee', 'ei' for connections and 'e', 'i'. I think numeric indexing is more general, so labels are mainly a human readable decoration. Maybe we can generate a mapping between the two?

Apparently the equivalent dictionary structure in C++ is called std::map. However, I've never used it.