Write field does not always execute/update

[jab0707]

I have noticed that when using write field to save out data to a .mat matlab field, if the network has not changed much then the write filed will not always reexecute/re-save the field out. I have confirmed this by deleting the saved out file then re-executing the network either by the big play button or the write field button. In either case the file is often not regenerated.

[dcwhite]

The needToExecute check there was simplified from v4 so probably doesn't check enough conditions. I can take a look in the next milestone.