Allow pointwise functions on array element

[vmiheer]

I think it should be possible to allow pointwise functions be applied to array element in expression.
E.g.

a = b * (sin(c) + d);

As of now the grammar doesn't allow this and causes syntax error.
Need to figure out how the addition would interact with existing optimizations.

1. sin(ID) could be though as just another ID.
2. Ideally in case of sin(expr), expr could be optimized.
3. Need to figure out if sin(SHIFTVEC) needs special handling.