New approach to array indexing

[richfitz]

Rather than the current minus1 approach (which seems quite fragile) go the other way and unpack matrices offset so that everything just goes.

This will affect

• all for loops, which will instead run from 1 to <= bound rather than 0 to < bound
• all functions that take a whole array (sum, multinomial distr)

but a large class of off-by-one errors will go away. When unpacking, rather than writing

double *x = odin_p->state[offset];

we'd just write

double *x = odin->state[offset] - 1;

and then later we can write x[1] and it will use the 0 offset of the actual state

[richfitz]

Not sure where this would leave 2d and higher arrays though

[richfitz]

should be fine - it's the same logic as one would use from R...