set the scales by setting the fourier periods

[Aaron O'Leary]

Allow this behaviour:

wt.fourier_periods = array_of_periods

The scales of a transform can currently be set by

wt = WaveletTransform(data)
wt.scales = array_of_scales

The equivalent fourier period is available as a property wt.fourier_periods,
however we can't set the scales via the fourier_periods. This would be useful
as the equivalent fourier period allows us to compare wavelets.