Initial guess for Global Registration

### 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

Hi!

I tested the global registration pipeline using FPFH and it works quite well!
My question; In my understanding the algorithm samples random points and the finds a transform that fits and checks optimum criteria. It would be great if one could introduce additional information into the algorithm, e.g. i know regarding one axis that the component must be in a specific rotation while regarding the other axes it's completely free. Is this something that could be included? At the first glance this limitation does not fit into the pipeline of the algorithm.
Another question regarding this: Does the initial relative position and orientation of the pointclouds to each other influence the result of the global registration?

Thank you!
Best regards
Christina