nest: Fixed handling of inhibitory weights.

Nest uses negative weight values internal to designate inhibitory synapses.

Currently, `projection.get("weight", ...)` returns negative weights if the projection is inhibitory. Updating the weights with positive values causes the synapses to become excitatory because the positive values not converted to negative ones before handing them to PyNEST. One has to specify negative weight values when updating inhibitory projection-weights via `projeciton.set(weights=...)`.

This update fixes both issues: Weights from inhibitory projections are returned as positive values and new weights written via `.set` are made negative prior to calling PyNEST.

There is also a unit test validating the behavior.

**Note**: This pull request might conflict with #289 because the line `value = make_sli_compatible(value)` is moved into the if-then-block; the resolution should be obvious.