Remove urdf from file names with macro definition

[galou]

Remove .urdf from file names of xacro files that only define macros.

Description

Rename .urdf.xacro to _macro.xacro.

Fixes #321.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Respect of conventions (breaking-change for people using xacro files that are maybe not part of the public API)

How Has This Been Tested?

Open in CROSS.

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation