Cropping meshes

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

I have created a custom capsule mesh using the default meshes. Now I wish to crop it.

1. I tried cropping it with the .crop(bbox) method, but it crops everything outside the bounding box. Is there a way to invert the selection and crop everything inside the bounding box?

2. I would also like to know if there is a way to crop one mesh using another mesh? So far, I have unsuccessfully tried finding the intersection of vertices of two intersecting meshes, and then remove the common points off one mesh. I have tried open3d.visualization.SelectionPolygonVolume's crop_triangle_mesh method, but I would like to use a simple method to crop the TriangleMesh.