Support for Pointcloud extensions

[chelm]

This PR adds support pointclouds as an item extension (https://github.com/radiantearth/stac-spec/tree/master/extensions/pointcloud).

Example usage:

item.ext.enable(pystac.Extensions.POINTCLOUD)
item.ext.pointcloud.apply(1000000, 'lidar', 'laszip', ept['schema'])

This work was done as part of STAC Sprint #6 and all tests are passing. Open to any suggestions or changes on this as well.

[chelm]

oh no my builds failed!?

[chelm]

oh flake issues, will fix

[chelm]

There we go!

[whatnick]

oh flake issues, will fix

I have been running ./scripts/test , got caught by flake8 a couple of times.

[lossyrob]

Thanks! This is looking good. There's a couple of small things mentioned inline.

There's a comment on considering whether or not statistics and schemas should be represented by their own objects. This happens in other extensions, and I think has a benefit of consistency and usability when constructing objects for properties. However I can also imagine some downsides, so happy to talk this through or lean on your experience as a user of the extension to decide which way to go with it.

One other thing - could you add a CHANGELOG entry? This feature will be released in 0.5.3 which currently doesn't have a section, so you could create one for this 'Added' feature.

[chelm]

Thanks @lossyrob for the comments. I'll get on them all this week as a i have time

[chelm]

Ok i've resolved the comments and this extension is feeling pretty good. Sorry it took me so long to get to this.

Now i just need to look at why the PR checks failed.

[lossyrob]

Looks great, thanks!