Access Info in gocean kernel uses strings instead of PSyIR

[hiker]

The stencil-based index information for a GOcean kernel is currently created out of strings, in gocean1p0._record_stencil_accesses:

i_expr = GOKern._format_access("i", i, current_depth)
j_expr = GOKern._format_access("j", j, current_depth)
var_accesses.add_access(signature, arg.access, self, [i_expr, j_expr])

which breaks dependency tools (and is in general just bad).

Fix this issue by creating PSyIR expressions instead.