Wavelet transform
The wavelet transform can now be run in parallel by passing `parallel=True`. This can provide a modest performance improvement of around 25%, along with an increase in memory usage.
Shared memory is used to reduce the memory usage, so parallelization only works on Python 3.8 and higher. (Parallelization is automatically disabled on Python 3.7 and below, even if `parallel==True`.)
Plotting
The performance of `pymodalib.contourf()` has been greatly improved by automatically subsampling the data. The subsampling can be disabled by passing `subsample=False`, and the resolution can be changed using the `subsample_width` parameter.
Additionally, `log=True` can now be passed to `pymodalib.contourf()` to apply a logarithmic scale to the y-axis.