Controls for tool initialization

[kkevlar]

The user currently has no control as to how the tool is initialized. Cassie.xml and stepdata.bin are automatically loaded as soon as the executable is run as the model/trajectory.

The user should have control of at least the following initialization options:

• Initialize the tool with any valid trajectory
• Choose how many times the trajectory loops when the timeline is created
• Initialize the tool with only a single pose
• Clip a segment of an initial trajectory file to loop over

Best case, the user could also drag models/trajectories into the window as is implemented in the original simulate.cpp which is bundled with the MuJoCo library.

[kkevlar]

Timeline initialization discussed here and this documentation will need to be updated through resolving this issue.

[kkevlar]

Merged in e90c1f7