Step 7 - Single PSM code string

[neworderofjamie]

This is the last syntax change I want to make. Previously, postsynaptic models had two code strings "decay code" and "apply input code" (the latter of which could also be configured via "custom converter code" in C++ for backward compatibility). This separation doesn't really make any sense and it's not obvious (if you'd asked me before I made this change, I couldn't tell you) what order they're run in. With this change:

exp_curr_model = create_postsynaptic_model(
        "exp_curr",
        decay_code=
        """
        inSyn *= expDecay;
        """,
        apply_input_code=
        """
        Isyn += init * inSyn;
        """,
        params=["tau"],
        derived_params=[....])

becomes:

exp_curr_model = create_postsynaptic_model(
        "exp_curr",
        sim_code=
        """
        injectCurrent(init * inSyn);
        inSyn *= expDecay;
        """,
        params=["tau"],
        derived_params=[....])

The user is no longer expected to know that Isyn is a magical thing you can only meaningfully add to (and can be renamed by setting post_target_var on the synapse group) and the injection of current uses the same syntax as current sources.

[tnowotny]

Yes, I think this is worth doing and makes things simpler and more sensible.