[WIP]nsxlib: Added sr latches
Both a NOR based and NAND based latch is provided with a _x1 and _x4 drive strength version for each.
Currently the check does not pass proof. It complains on logical loop inside the _ext.vhd file. I need help on getting this through sign-off check. I tried two styles for the .vbe file, one for the srlatch* cells and another nsnrlatch* cell but the problem seems to be with the extracted netlist.