Possible improvements for ModelBuilder scripts

[carmichaelong]

#999 does a good job of simplifying the interface for model building via GUI scripting. Some suggestions for further improvement:

1. Add other methods to wrap current ones to remove the need to pass in a model.

• I think users with less coding experience may have trouble with thinking about getting a model object and passing it in correctly.

2. Make it less error prone to invoke loadModel().

• I think some users will want to use loadModel() a lot to visualize incremental changes at they work on a model. We could make sure we either call it for users, or make sure it can only be called when it won't fail.
• For example, currently in the GUI, adding a Body without a joint will cause loadModel() to fail.