Bug fix: properly partition networks containing one-sided gap-junction connections (#1774)

Replace the `partition_load_balance` function assumed that gap-junction connections are always double-sided (i.e. if gid `x` has a gap-junction connection from peer gid `y`, then gid `y` must also have a gap-junction connection from peer gid `x`). This used to be a requirement that was checked prior to #1682. Since then, single-sided gap-junctions are in principle allowed, but `partition_load_balance` still operates under the bidirectional gap-junction connection assumption resulting in some gids being present in multiple cell-groups. 
This PR modifies the `partition_load_balance` function to do the following: 
1. On each rank, generate a gj_connection list per cell. (gj_connection list is a vector of gids that have an outgoing connection ending at the cell under consideration). 
2. On each rank, gather all the gj_connection lists for each cell in the network. 
3. On each rank, modify the global list of gj_conne...