reproject_interp missing method description or reference

[dstansby]

The docs reproject_interp (https://reproject.readthedocs.io/en/stable/api/reproject.reproject_interp.html#reproject.reproject_interp) are currently missing an explanation of how the method works, or a reference to somewhere else that explains how the method works.

[Cadair]

closing as a duplicate of #237

[dstansby]

237 is for reproject_exact - this is one for reproject_interp!