[Moco] Support optimizing properties nested inside other properties

[nickbianco]

Related forum post here: https://simtk.org/plugins/phpBB/viewtopicPhpbb.php?f=1815&t=18272&p=50217&start=0&view=

For example, the stiffness property for an ElasticFoundationForce is stored inside the another property: contact_parameters. Moco currently does not support optimizing such properties.