Some functions added

[migperfer]

Reference Issue

No reference issue

What does this implement/fix? Explain your changes.

Implement a function that were not wrapped: frequency multiply: To multiply all frequencies by a certain factor, helpful with some stuff that shifting by semitones cannot achieve. Is the -f option in the CLI

tempo shift: Instead of calculating manually the ratio conversion for the tempo, use the function already included in rubberband

[bmcfee]

Thanks for doing this! It appears that the CI build is (mostly) failing due to changes in the pytest API since I last worked on this package. We should address those in a separate PR, but in the meantime I'll leave some comments on this one.

[bmcfee]

Quick update: I've fixed the pytest issues, so if you could rebase to the current master, the travis build should work again.