Swap gates are incorrectly rendered in LaTeX

[hodgestar]

The function latex_code() in circuit.py does:

col.append(r" \qswap \qwx[%d] \qw" % distance)

but the qcircuit package expects this to be \qwx[-%d] -- note the minus sign. This causes the pdflatex compilation to fail if the swap goes below the circuit (at least recent enough versions of qcircuit throw an error).

See https://mirrors.evoluso.com/CTAN/graphics/qcircuit/qcircuit.pdf for the qcircuit documentation.