NewModel command to clean up any unused labels in the model after initialising it.

#### Description:

The NewModel command should clean up any of the unused labels stored in the model from previous open sessions. Labels are carried over between models by default for Robot, which can lead to slightly un-intuitive behaviour, especially if a model with a significant amount of for example sections and spring supports have been opened before a new model is created.

This can lead to some oddities when using the BHoM when you might think you have a clean model, with no sections in, but there still exists remenants from another project in there.

Clearing out all the labels when using the command resolves this issue. If the engineer want the labels to persist, then the normal usaga of "NewModel" via the Robot UI still provides this option.