Before running the simulation, check if necessary arrays are copied to GPU

[esinyavuz]

Silent errors occur when we forget to copy a variable to the GPU. It would be useful to warn the user if:

• the values in the GPU are not meaningful (in general arrays contain only zeros if allocated but not initialized)
• necessary GeNN functions are not called

[esinyavuz]

One way of doing this could be to add a "sanity check" function in runner.cc which would go through the list of all the neuron and synapse groups, copy their variables from the device, print the first values, and sum of absolute values, and return a warning if this sum is zero. Users can then call this function before calling the kernels if they wish. It should be easy enough to implement.

[tnowotny]

This has been overtaken by history.