Speed up resample_melody_series

[rabitt]

Originally resample_melody_series would resample no matter what, and this caused things to be slow when calling melody metrics many times on small segments (my use case).

Now, resample_melody_series first checks if the timebase is the same, and if it is simply returns back the original arrays rather than going through the resampling.

[craffel]

Nice, thank you! You are in luck, because doing the docstring audit for 0.4 is taking me forever, so we can still squeeze this into 0.4. Can you

1. Add a note to the docstring which says that times and times_new are equivalent, no resampling will be performed
2. Add a note to the changelog?

Thanks!

[rabitt]

Done!

[craffel]

LGTM, merging!