[Unity plugin] Support multielement springlength in tendons

[Balint-H]

A quick small fix for this missing feature.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[yuvaltassa]

This LGTM but I'll let @erez-tom review.

More broadly, I propose the following easy test: Load all the models under mujoco/test/..., complain if any don't load. Would this or a similar test catch future bugs where a feature is added to MJ but is missing from the plugin? When adding a new thing, we generally always make models to test it, and they go somewhere under that directory.