Python .instances() appears to be shared among running jupyter notebooks

[slochower]

DAT_restraint.instances() is shared inside jupyter kernels somehow.

For example, two notebooks can share the same DAT_restraint.instances() even if they have separate restraints defined inside of them.

This means DAT_restraint.instances() may not be reliable.

[slochower]

We should no longer be relying on DAT_restraint.instances(). Closing.