Bugfix: Align calculation of number of ghost layers on GPU with CPU version

For the calculation of the number of ghostlayers only relative accesses
should be considered like on the CPU version
8 jobs for bugfix-ghostlayers-gpu in 5 minutes and 30 seconds (queued for 1 minute and 23 seconds)
Name Stage Failure
minimal-sympy-master Test
File "/builds/seitz/pystencils/pystencils/transformations.py", line 526, in visit_node
sub_ast.lhs = visit_sympy_expr(sub_ast.lhs, enclosing_block, sub_ast)
File "/builds/seitz/pystencils/pystencils/transformations.py", line 504, in visit_sympy_expr
field_access.offsets, field_access.index)
File "/builds/seitz/pystencils/pystencils/astnodes.py", line 591, in __new__
assert isinstance(base.label, TypedSymbol)
ERROR: Job failed: exit code 1