[Bugfix] unstable hybrid robot example

[zswang666]

Previously, contact_resolve_time was hardcoded to 0.02 but now it is set to None with default value as twice the substep_dt in the rigid solver. In hybrid robot setup, the dt's of the rigid and the mpm solver are set to the same value to avoid drifting in coupling. However, this makes the contact_resolve_time becomes super small, leading to a stiff system and large impulse. Thus, we need to set the contact_resolve_time to a larger number (0.02) to fix the issue.

Related issue: #476