refactor stencil index offsets
refactor stencil index offsets

sklam/numba,jriehl/numba,gmarkall/numba,numba/numba,stuartarchibald/numba,seibert/numba,IntelLabs/numba,IntelLabs/numba,cpcloud/numba,jriehl/numba,numba/numba,jriehl/numba,sklam/numba,jriehl/numba,gmarkall/numba,stuartarchibald/numba,stuartarchibald/numba,IntelLabs/numba,stuartarchibald/numba,IntelLabs/numba,numba/numba,cpcloud/numba,stonebig/numba,stonebig/numba,cpcloud/numba,seibert/numba,sklam/numba,stonebig/numba,IntelLabs/numba,numba/numba,stonebig/numba,sklam/numba,numba/numba,stuartarchibald/numba,seibert/numba,gmarkall/numba,cpcloud/numba,sklam/numba,gmarkall/numba,seibert/numba,gmarkall/numba,seibert/numba,cpcloud/numba,stonebig/numba,jriehl/numba