CODE IMPORT: copy intervals code from msprime

The authors of this module and associated tests have agreed to release
the code originally developed for the msprime package under the MIT
licence (see #2636). Specifically:

@jeromekelleher: "I approve the relicensing of this code originally
developed in msprime from GPLv3 to MIT."

@hyanwong: "I am happy to relicense all of my contributions to this code
(originally developed in msprime) from GPLv3 to MIT."

@awohns: "I also approve the relicensing of this code originally
developed in msprime from GPLv3 to MIT."

@grahamgower: "I approve the relicensing of this code originally
developed in msprime from GPLv3 to MIT."

@petrelharp: "I approve the license change!"

Author: jeromekelleher