Improve documentation for interpolate function

[natema]

The docstring of interpolate currently cite a non-existent fourth argument ("an increment value").
This should be fixed, and the docstring should be expanded to include a clear explanation of what the function is doing.
At the end, it should also cite the DYNAMO manual for reference:

A. L. Pugh, DYNAMO user’s manual. 1963. Accessed: Jun. 02, 2022. Available: http://archive.org/details/dynamousersmanua00pugh

For convenience, I'm copy-pasting the screenshot of the pages of the manual that refer to it.