Fix title in autocorrelation plots
Fix off-by-one bug in trapezoidal quadrature rule

Integration of constant functions is correct now:

>>> qr = TrapezoidalQR(-2.0, 5.0, 13)
>>> QR = TensorProductQR([qr, qr])

>>> NO = QR.get_nodes()
>>> no1, no2 = vsplit(NO, 2)

>>> om = QR.get_weights()

>>> g = lambda x,y: 3.2

>>> sum(g(no1, no2)*om)
156.80000000000038

>>> 7*7*3.2
156.8

(Note that the trapezoidal qrule was never used 'in production')