exact imaginary time shift invariance in interpolation

[kleinhenz]

Use exact expressions $G(\tau_1, \tau_2) = G(\tau_1 - \tau_2, 0)$, $G(\tau, 0) = \xi G(\tau + \beta, 0)$ in imaginary time interpolation for ImaginaryTimeGF, FullTimeGF, and TimeInvariantFullTimeGF.