Add warning for bad choice of wavelet width

[jrudoler]

It's easy enough to check in the filter method of MorletWaveletFilter that the combination of width, freqs, and length of eeg epoch is valid.
To be explicit, we want:
2 * width * 1 / min(freqs) <= len(timeseries.time) / samplerate
in words, the length of time must be at least twice the width of a wavelet at the lowest frequency.