add transient F(t0,tau) file output

This adds two bits of transient output:

1. two extra columns for the maximum-likelihood t0,tau values to the standard gridSearch output file, iff a transientWindowType was set
2. one extra file per Doppler candidate with the full t0,tau grid iff a new explicit option outputTransientFstatMap is set to True.

The main low-level change is that get_fullycoherent_twoF now returns 3 results, two being None by default except for the transient case, and so I added [0] to its calls from other usecases. Tests and examples seem to be unaffected, so I hope I got all of them, but a double-check can't hurt. Or maybe you now a more elegant solution @GregAshton ?