[BBPBGLIB-1042] GapJunctionManager: filter source target by population (

#42)

## Context
Currently neurodamus raises an error when the connection source or
target `node_set` from multiple populations. Users should use
single-population `node_sets` in `connection_overrides`. This also
happens for `GapJunctionManager` when we have a multi-pop `node_set` for
the simulation (`circut_target`). This PR fixes that by limiting
"_src_target_filter" from the gap junction's source population so that
it belongs to a single population.


## Scope
Change in class `GapJunctionManager:__init__`

## Testing
Existing test

## Review
* [x] PR description is complete
* [x] Coding style (imports, function length, New functions, classes or
files) are good
* [ ] Unit/Scientific test added
* [ ] Updated Readme, in-code, developer documentation

---------

Co-authored-by: Fernando Pereira <fernando.pereira@epfl.ch>

neurodamus/gap_junction.py

@@ -86,7 +86,8 @@ def __init__(self, gj_conf, target_manager, cell_manager, src_cell_manager=None,
             raise ConfigurationError("Missing GapJunction 'Path' configuration")
 
         super().__init__(gj_conf, target_manager, cell_manager, src_cell_manager, **kw)
-        self._src_target_filter = target_manager.get_target(cell_manager.circuit_target)
+        self._src_target_filter = target_manager.get_target(cell_manager.circuit_target,
+                                                            src_cell_manager.population_name)
 
     def open_synapse_file(self, synapse_file, *args, **kw):
         super().open_synapse_file(synapse_file, *args, **kw)