Added fftTime() plotting method to Trajectory
SpectralTrajectory subclass

Also created grid class as a master class of
spectralGrid.
And Trajectory.fftTraj() method (the whole
point of this commit) to follow evolution
of trajectory spectrum.
Renamed SpectralGrid -> PeriodicGrid

(there was now a confusion with the
SpectralTrajectory class... which has a non
spectralGrid...)
Release v1.0.1 : spectral trajectories