pass correct index to the NMODL procedures (#610)
pass correct index to the NMODL procedures (#610)

Fixes an error in vectorized kernels that sees the incorrect index passed to PROCEDURE calls.
The loop index variable was being passed, instead of the pack of vector indexes.

Fixes #609
halfflat/arbor,eth-cscs/nestmc-proto,eth-cscs/nestmc-proto,halfflat/arbor,halfflat/arbor,halfflat/nestmc-proto,halfflat/arbor,halfflat/nestmc-proto,halfflat/nestmc-proto,eth-cscs/nestmc-proto,halfflat/arbor