Support `add_frame` directives when adding frozen children

[siddancha]

Fixes #322

Solution

• [x] Implements a new DirectivesTree class in manipulation/directives_tree.py. This class creates a tree where:
  • Nodes: are defined by (1) model instances and (2) added frames.
  • Edges: are defined by (1) add_weld and (2) add_frame directives.
• [x] The tree implements two recursive search functions:
  • Finding paths from the root ("world" frame) to a given set of robot instances. The use case is to find the minimal set of directives that supports the given robot instances.
  • Finding paths from given robot instances to leaves. The use case is to find child model instances that are welded to the robots (as well as accompanying directives), which is in-turn used to lock the childrens' joints and add their directives to a controller plant.
• [x] This abstraction simplifies the code in _PopulatePlantOrDiagram().

Unit testing

• [x] Added a test file: manipulation/test/test_directives_tree.py that tests the individual search member functions of DirectivesTree at the low level and MakeHardwareStation at the high-level.

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This change is 

[RussTedrake]

Nice! I'm up to my ears in some work at the moment. @nepfaff ... could you take a first look?

[nepfaff]

I'd say its ready to squash + merge (only tiny nit comments left). @RussTedrake should we go ahead or do you want to have a final look?