Check if removing `i` and `j` breaks indexing

[denisalevi]

We remove i and j when they are not used in synapses code here. But as @mstimberg mentioned, this can break indexing other arrays with i and j, which we are not testing for.

Check if this is breaking things, i.e. run some code that doesn't use i in synapses but for initialisation (e.g. syn = Synapses(...); syn.param['i<10'] = 5.

Can we check for those cases and still remove i and j from device at least (for indexing the result arrays in python, it would be enough to keep them on the host, which we currently do anyways).

[denisalevi]

I ran into the problem of deleted i/j during some tests that used multiple run calls and set synapse variables inbetwen. In commit 4ea95d2 (large commit with multiple fixes...), I implemented a fix that check whether i/j are used anywhere after the first run call and if so it does not delete the arrays at all (they might be needed on the device to reinit synaptic variables).

Things left to do here:

• I am not sure if this differentiates between needing i/j on host or device. I think what I implemented only checks if we need them on the device. Double check this.
• i/j are still deleted if they are only needed in the first run. This might give a problem for the example I stated above (no i/j in synapses code but in the param initialization, for only single run simulation).
• I didn't look ino the case where we need i/j only on Python side after the run (e.g. for indexing the array with pre/post indcs). Here we should be able to just keep the indcs on host and delete them on the device I guess.