Lazy loading of points and spatial index

[kevinyamauchi]

@LucaMarconato has observed that load lots (1E6) points is slow. Thus, we should discuss lazy loading and using some sort of spatial index for query.

A few points:

• we need to define the access patterns we want to support and prioritize which should be fastest
• we need to determine if delayed loading is sufficient or if we need a spatial index
• we should separately consider analysis and rendering cases
• we need to decide if spatial index should be able to be saved/loaded

[LucaMarconato]

Lazy loading is now supported. Spatial index could be achieved using Dask GeoDataFrame for points. We are still considering this.

[LucaMarconato]

• [ ] after this is implemented we should explain how to use it in the docs.

[melonora]

Since #359 contains the information of the remainder of the work I will close this issue.