Allow Multiple Frames for Attachments

[mbrauckmann-bdai]

Is your feature request related to a problem? Please describe.
We are working on a bi manual setup with two franka arms. in order to plan for both arms simultaneously and avoid self collisions, we would like to create a single robot in vamp with both arms. However vamp currently only supports one frame for attachments so we would not be able to pick up objects with both arms and have those objects included in the plans.

Describe the solution you'd like
Ability to specify the frame/ robot link that an attachment should be fixed to.

[zkingston]

@mbrauckmann-bdai we are working on this internally - no concrete timeline right now but expect this in the near future. We also have a bimanual Franka setup we are testing on.