Order of operations in STDP with delays

[thesamovar]

If the user gives something like:

pre= x += 1 w += x*y y += 1

In STDP with delays this will give the wrong results, because the x+=1 and y+=1 operations will be scheduled to execute immediately, and the w+=x*y operation happens with a delay. We should raise a warning in this case.

[thesamovar]

A warning is now raised including (for CSTDP) when this occurs without delays. In some cases where a warning is currently raised this could be supported, but it needs some substantial rewriting. I suggest we leave it until the code generation rewrite takes place anyway.