Standardize and make clear when to unique-ify cell names

[clavin-xlnx]

Vivado generally exports DCPs that have a one-to-one mapping between cells and their instances. Vivado will unique-ify cells so that this is the case. However, not all designs that come from Vivado have this attribute. Therefore, when combining designs in RapidWright, there can be unclear scenarios when to force the invocation of unique-ifying cell names in the netlist. Currently this is done with an optional parameter in EDIFNetlist.migrateCellAndSubCells(). However, it is unclear to the user when this would be required.

The goal of this issue is to achieve a solution such that the user no longer needs to worry about unique-ifying cells, RapidWright should detect and correctly handle the situation for them.