Loss of color when converting to open3D

**Describe the bug**
When converting to an open3d instance, the color of the point cloud is lost

**To Reproduce**
Steps to reproduce the behavior:

```python
import open3d
from pyntcloud import PyntCloud

pointcloud = open3d.io.read_point_cloud("MyColorPointCloud.ply")
open3d.visualization.draw_geometries([pointcloud])
# See screenshot below

pyntcloud = PyntCloud.from_instance("open3d", pointcloud)
pointcloud_reconverted = pyntcloud.to_instance("open3d")
open3d.visualization.draw_geometries([pointcloud_reconverted])
# see screenshot below
```

**Expected behavior**
The color should be preserved through `to_instance` call.

**Screenshots**
First visualization:

Second visualization:

**Desktop (please complete the following information):**
- OS: Windows 10
- PyntCloud Version: 0.3.1

Start with the implementations behind PyntCloud.from_instance("open3d", pointcloud) and pyntcloud.to_instance("open3d"). Reproduce the supplied round trip with the color PLY and compare both Open3D visualizations; done means the reconverted point cloud preserves its original color.

Written by the indexing model from the issue text.