remove usage of get_connection?

[mgeplf]

I can't tell if this note is true, and whether get_connection was removed.

Note: Due to multi-population, a connection may not be unique
        for a given pre-post gid pair. As such get_connection() doesn't
        make sense anymore and this method shall be used instead.

[mgeplf]

From:

neurodamus/neurodamus/connection_manager.py

Line 450 in 7b9917f

Note: Due to multi-population, a connection may not be unique