check friction in io.put_model

[thowell]

instead of silently overriding, provide a warning if small friction values are provided

[adenzler-nvidia]

I'm not sure about existing conventions, but it could be an option to override with a warning, and avoid the potential NaN in all cases.

But this looks good as well, I'm just not sure what is consistent behavior.

[thowell]

the friction values are clamped here with MJ_MINMU (=1e-5).

the issue is that this minimum value was tuned for double instead of float and this value seems to be insufficient to prevent nans, see experiments with mujoco using floats here.

opened an issue to track updating relevant constants #1000.

[adenzler-nvidia]

approving, yes makes sense to warn for this.