Allow Paulis in Checks to have sign -1

[OzYossarian]

For v0.0 will only allow +1 signs. If -1 signs are allowed, can implement this by either flipping the measurement outcome of the whole check if an odd number of Paulis have sign -1. Or even by directly measuring -1 signed Paulis, e.g. changing rotation gates.

[OzYossarian]

I got this wrong - it should be a syndrome extractor that handles what Pauli letters and signs are allowed, not the Check. In a Check, the only constraint should be that the total product is Hermitian, which we've now implemented.