Refactor: Location of trace norm should be changed

[vprusso]

At present the trace_norm.py file is located in the state_metrics/ directory. Generally, the other metrics that are contained within the state_metrics/ directory are some type of distance measure between one state and another. The trace norm function breaks this pattern and is really just a measure of the state itself (instead of being a measure of a state against another state).

It may be more appropriate, therefore, to move the trace_norm functionality inside of the matrix_props/ directory. This would also seem to make somewhat more categorical sense, as sk_norm.py is contained in matrix_props/ as well.