Parameterize SDF files as opposed to dynamic spawning

[mabelzhang]

Currently, worlds are dynamically spawned in the test code (e.g. boxes.hh, boxes_dt.cc, boxes_model_count.cc).
This couples world definitions to simulator-dependent tests.

To allow tests to be created for other simulators, the world definitions should be decoupled from the test code.
Ideally, the worlds should be defined in documentation, which is then implemented for each simulator. This could be in SDF, if multiple simulators are able to load SDF files, or in different file formats suitable for each simulator.

What parameters to choose:
For future extension to other simulators, it may be useful to have two sets of parameters for some or all of the tests.
Default: There should be a default set of parameters, which is not overfit to tuned parameters for any particular simulator or test.
Best case: A "best case" set of parameters can be optimized for each simulator / physics engine.

Starting point: the world is partially parameterized in https://github.com/scpeters/benchmark/blob/master/worlds/boxes.world.erb
See boxes.hh for other parameters.