Allow Multiple Frames for Attachments

**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.

Start by reviewing linked pull request #75 and then locate the attachment-handling entry points; the issue names no files or tests. Confirm how an attachment's robot link is selected, and verify that separate attachments can use both arm frames and be included in simultaneous planning.

Written by the indexing model from the issue text.