Move thread tiling initialisation into grid initialisation

[arporter]

Currently each field object computes and stores its own thread tiling. There are two issues with this:

1. there's a lot of duplicated work
2. more seriously, we don't yet have a thread tiling when creating (and initialising) the T-mask inside the grid object. That means we can't use first-touch to ensure it is stored in memory that is local to the thread that will be accessing it.

In this issue we will look to move the thread tiling setup into the Grid initialisation.