use data.shape in Qobj.shape
added state functions for excitation-number restricted states
added function for annihilation operators for modes in excitation-number restricted state spaces
minor cleanup. use shape of underlaying data only if data is set, otherwise fall back on computing shape from dims
make sure that Qobj.shape returns a list rather than tuple
updated docstrings for enr functions
docstring spell fix
unit tests for enrs functions
consistently use shape rather than dims when reshaping Qobjs
optimize state_number_enumerate for case with few excitations in a big hilbert space
added enr_identity and enr_thermal_dm. Based on code contributed by Neill Lambert (@nwlambert).
moved enr_identity to operators, added functions to __all__
added unit tests for enr identity and thermal_dm
Merge pull request #262 from jrjohansson/feature-excitation-number-restricted-states

Feature excitation number restricted state spaces