add stencil access that support dynamic offsets given some constant bound

[gysit]

add stencil accesses that access an array a dynamic offset. We may still may want to provide static bounds somehow to enable shape inference. Ideally the bounds should be defined for the accessed fields and probably not local relative to the current position.

[gysit]

example syntax:

%0 = stencil.dyn_access %in(%i, %j %k) in ([-3, -3, 0]:[3, 3, 0]) : (stencil.temp<?x?x?xf32>) -> f32

• provide a dynamic offset
• the range specifies the absolute range of the dynamic access not considering a possible constant shift due to stencil inlining
• the offset may be an optional attribute set to zero by default

open questions:
does it make more sense to specify the access range using absolute or relative offsets?