Explicit timestamps as indices

[jaidevd]

By default. if timestamps of a signal are not provided, integer indices are used. When they are provided, they cannot be arbitrary timestamps since the signal is sliced using timestamps directly, as follows:

self.tfr[indices, self.ts] = self.fwindow[lh + tau] * self.signal[self.ts + tau] * \
    np.conj(self.signal[self.ts - tau])

This needs to be fixed by converting the timestamp indices to intermediate integer indices, and using them to slice the signal.