Add simple_humanoid

[talos_reduced] Fix inertia and mass to zero for optical links.

ref #67

In case we actually have a 1.0kg body, we can still put its name into
the one_kg_bodies arg.

Letting this unfix is adding 1kg for each link in pybullet.

Define unknown materials to hide URDF parser warnings

Fix Anymal SRDFs

Added conda info in README

report warnings to end users, fix #60

python: allow loading of multiple robots

RobotLoader

Remove lights from Anymal robot

python: fix double negative