Calculate offset to reduce the area for safe placing

[jarkenau]

Currently the offset for reducing the place area (e.g top table surface) to a safe sub-area is hard coded:
https://github.com/aprilprojecteu/grasplan/blob/c8a27e6fa6685c20a441b1e61b4aeb7fc40634b1/src/grasplan/place.py#L205

We often need to adjust it for demos, further it also highly depends on the current object in the gripper and computing it based on that shouldn't be to difficult. However the different "upwards" axis described in #10 could require some special cases.

[mintar]

Also, this line has two bugs. :)

I've fixed them in #13.

[oscar-lima]

Idea: offset the place plane using interactive markers