Add interpolation module

[brendancol]

• [ ] General concept implemented here: interpolation.idw() method (checkout implementation here: https://github.com/makepath/safegraph-where-should-i-shop/blob/master/SafeGraph%20-%20Where%20Should%20I%20Go%3F.ipynb)
• [ ] Add Basic Numpy / Numba support
• [ ] Add Dask Support
• [ ] Add CuPy Support
• [ ] Test against QGIS
• [ ] Make sure code is well tested
• [ ] Add to Documentation
• [ ] Add to user guide

[giancastro]

@brendancol, will this issue be worked on?

[DahnJ]

@brendancol does the linked notebook actually contain the implementation? The notebook refers to an "external module":

import idw # external module located next to notebook

but there's no such module in the repository.