Force 60fps rendering in pendulum, mass-spring

This change to modelSampleRate causes the model to use setInterval instead of requestAnimationFrame. I noticed that sometimes Chrome selects a lower rAF rate (30fps or even 15fps) after switching from external monitor to internal display--even though using setInterval easily achieves 60fps.

For the InquirySpace models it's important to try to run at real time if possible.

IS [#43959831]