__len__() for coordinate systems

**Current Problem**
Sometimes it would be convenient to call `len(co)` for a coordinate system `co` to give the number of qubits in a fully-yielded graph of that type, for example when we are passing the coordinates object but not other parameters.

**Proposed Solution**
Writing __len__() methods for `zephyr_coordinates`, `pegasus_coordinates`, and `chimera_coordinates`