Support for finding the nearest interval.

[rLannes]

Hi, First, thank you for implementing this library.

It is really nice to have a fully self-contained Python library => No Issue with dependency love, wave, overly happy smile
I plan to use your library for genomic interval analyses.
It only misses one feature, the ability to find the nearest interval from an interval that doesn't have a match on the Tree.

I will implement it, but I am more a data analyst than a coder.
Do you accept merge request?

Having it in the library would be very enjoyable.

Best