Handle streams in fpga_transform_state

[TizianoDeMatteis]

Currently, streams are not properly handled in fpga_transform_state. This will let the codegeneration phase fails in one tries to convert to FPGA DaCe programs that contain stream (e.g. samples/simple/filter.py).

In the case of transient streams, this could be fixed by changing the storage class (+ I think some changes in sdfg_nesting.py).