Add CSS.get_destabilizer_ops

[perlinm]

Cleanup of #543. The example (copy-pasted and modified) is now:

Example usage: I have a syndrome vector syndrome_bits = (s_0, s_1, ...) from stabilizer measurements. I want the destabilizer that zeros out that syndrome and commutes with all logical operators. Say the code is self-dual and defined by a minimal set of stabilizer generators. I can construct the desired stabilizer with some thing like:

from qldpc import codes
from qldpc.objects import Pauli

code = codes.SteaneCode()
syndrome_bits = (0, 1, 0)

# X-type stabilizers and their Z-type duals
stabs = code.get_stabilizer_ops(Pauli.X)
destab_ops = code.get_destabilizer_ops(Pauli.Z)

syndrome_vec = code.field(syndrome_bits)  # convert to a galois.FieldArray
print(syndrome_vec)

net_destabilizer = syndrome_vec @ destab_ops
print(net_destabilizer)