Change method for spawning agent random grid positions

[zeyus]

Currently uses an NP array with a shuffled sequence of all the IDs and then reshapes it to the grid size...
It works fine but this part seems to have quite an overhead on the initial loading

1. evaluate performance overhead (it's only once per simulation, so there might be better places to optimize e.g. stuff inside the sim)
2. if a problem, refactor, this will benefit initial sim loading and especially multi-run speed.