list all cells used

[olerichter]

This adds the info of which cells are used by a design,

Ideally to be even more useful it should print how often each cell appears in the hierarchy,

to do this dumbly (and inefficiently) I could first rise in the hierarchy for the first cell with getParent then on the top node traverse all nodes. When a UsrType says it's a leaf -> ask it if it's a process, then ask it if it's a cell, ask it which cell of the list, and increment a counter. There might be a more efficient way.

[rmanohar]

I am going to merge this in, but will update the command itself to have an option (-l) that will then list the cells.