Adding Basic QSD Decomposition to BQSKit

[jkalloor3]

This adds a naive, un-optimized version of Quantum Shannon Decomposition to BQSKit. This pass recursively breaks down a unitary of n qubits to 4 unitaries of width n-1, until you reach unitaries of a desired size (usually width 2). The pass also has the option of running ScanningGateRemoval (with the ability to do TreeScan) to try and remove unecessary gates at each level of the decomposition.