Add support for saving spheres to SRDF

[nickswalker]

Old ROS docs
MoveIt parser

Add support for saving spheres to Semantic Robot Description files. Add ability for spherize_robot.py to load a URDF from a file path (as alternative to robot_descriptions), and optionally output SRDF. I haven't touched the other scripts, but it might be nice for some of them to support the SRDF standard as well.

SRDFs are semantic metadata files meant to accompany URDFs and clarify how to use the URDF for e.g. planning. The spec includes link sphere approximations.

Example:

<?xml version="1.0" encoding="utf-8"?>
<robot name="fr3_robotiq">
	...
	<link_sphere_approximation link="fr3_link0">
		<sphere center="-0.012898551136139233 -0.0012041360684210938 0.06767921685539421" radius="0.11959"></sphere>
		<sphere center="-0.08394655113613923 0.0038248639315789064 0.04058421685539422" radius="0.095099"></sphere>
	</link_sphere_approximation>
	<link_sphere_approximation link="fr3_link1">
		<sphere center="0.0002572757452726364 0.00036496158409118906 -0.1395377861636877" radius="0.076596"></sphere>
		<sphere center="-0.0002227242547273636 -0.05429003841590881 0.0004042138363122977" radius="0.077209"></sphere>
		<sphere center="0.0021852757452726366 -0.032367038415908815 -0.07320478616368771" radius="0.063841"></sphere>
	</link_sphere_approximation>
	...
</robot>

Very parallel with CoMMALab/foam#17