Add fmin and fmax to freq_vector_log()

[neillinehan]

Is your feature request related to a problem? Please describe.
I would like to examine specific scale ranges with wavelet coherence, tuning the minimum and maximum frequencies can allow more fine analysis, reducing unnecessary computation, especially with the wwz method.

Describe the solution you'd like
Add fmin and fmax to allowed arguments for freq_vector_log(), and the others.

[CommonClimate]

Hi @neillinehan,
I've implemented this feature and the custom ntau/tau, and tested both in a Jupyter Notebook. It works and it's actually a nice addition to our bag of tricks, so thank you for suggesting it.
Please pull from the master branch and give it another try; if it works for you, please close the issue.
Best,
Julien

[CommonClimate]

Closing this now since I have not heard word from @neillinehan . Can be re-opened if need be.