Fix IO unit for array_io module

[cmacmackin]

Currently I specify that the array_io module just uses one unit (90) for everything. However, if the user decides to use this unit for something of their own then they will get an error. Additionally, this means that none of the IO in this library can be done from within parallel regions in the code (not if you're using shared-memory parallel architectures, anyway). This should be fixed at some point.