Added ```GPUArray.__pos__```
- Was needed to pass https://github.com/inducer/arraycontext/blob/fd4d4d8bff8b4f5ae0b3960c7c9b021066ff388e/test/test_arraycontext.py#L535
- Added
test/test_gpuarray::test_unit_multiply_array. - Needed for kaushikcfd/arraycontext!1