It is not obvious how to retrieve named plugins from the context or session after launch

[eirrgang]

An undocumented feature allows access to the object providing MD extension code by exposing an array of the plugins in a simulation through the Python gmx.Context. (This is one reason we haven't allowed multiple simulations in a graph yet.)

Registered plugins appear in the array in the order in which they were added as dependencies to the MD simulation operation, but there is not a good way to map between the name given to an operation and the element in the list.

Workaround:

1. Keep track of the order in which plugin WorkElements are passed to add_dependency
2. Add bindings in the plugin for the name in the restraint object.

Planned solutions:

1. primarily, resolve Add proxy access to data graph through WorkElement handles #94 (subtask of Reorganize Context and WorkSpec hierarchy #88, has some dependencies)
2. make progress on schema 0_2 #65 wrt named and typed input/output "ports"

[eirrgang]

Reported as a result of conversation with @jmhays. Please post if there are updates or corrections. Feel free to comment about urgency and workarounds.