Many beam joints

[obucklin]

This changes the DirectJointRule to accept a list of elements instead of only 2, allowing us to have joints with more than two elements. This includes a new GH component, JointRuleFromList which makes a single joint from the list of elements. A GH data tree can be used and each branch will create a single joint with the elements contained on that branch.

Beam elements are now added to joint.elements.
Joint objects generate one or more interactions via Joint.interactions.
There are some other minor changes, too.

• Bug fix in a backwards-compatible manner.
• New feature in a backwards-compatible manner.
• Breaking change: bug fix or new feature that involve incompatible API changes.
• Other (e.g. doc update, configuration, etc)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I added a line to the CHANGELOG.md file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• I ran all tests on my computer and it's all green (i.e. invoke test).
• I ran lint on my computer and there are no errors (i.e. invoke lint).
• I added new functions/classes and made them available on a second-level import, e.g. compas_timber.datastructures.Beam.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)

[obucklin]

@chenkasirer @papachap this is ready for review