Improved Timeseries unit tests
This PR introduces a few more unit tests for the TimeSeries
object, for the following instance methods
fft
average_fft
spectrogram
spectrogram2
The methods unfortunately don't yet check that the results are sane, just that the methods don't raise exceptions, or do raise exceptions where expected. This has already revealed two bugs whose fixes are included with this PR - this fixes #77 (closed).