class pq_string: describes all aspects of a string of operators / tensors and includes some functions to query normal order, set amplitudes/integrals/spins, print, etc.
class pq_tensor (and derived classes amplitudes / delta_functions / integrals): labels for tensors, functions to sort, print, etc.
pq_utils.cc: most of the methods for bringing a pq_string to normal order and also for consolidating terms / identifying permutations, etc. once a list of normal operators has been generated
pq_swap_operators.cc: functions for swapping operators in a pq_string toward normal order (wrt true or fermi vacuum)
class pq_helper: python interface functions. all add_xx methods end up running through add_operator_product, which translates the python input into a pq_string and then calls the logic to bring that pq_string to normal order
pq_extras.cc contains some functions that are not currently used but could be useful in the future
Also, fixes #33 and adds tests for ccsd and ccsdt with spin
Cleans up structure of C++ code
Also, fixes #33 and adds tests for ccsd and ccsdt with spin