Improving performance and robustness

[rogervankanten]

• Code has been made more robust by using context managers
• Optimizations run much faster by loading MuJoCo model from XML only once
• All example conversions were performed again and model output was updated

[HuaweiWang]

Yes, I checked them. The converted models have similar results as before. I think Roger implemented an initialize function so that multiple instances will be generated for parallel computing.