Fix ItemIdentityException

[corycrean]

This PR fixes an ItemIdentityException I encountered while working through this tutorial (more details here).

I found that two different materials were being given the same name. It looks like the material name gets set in two places. Here, the name gets created by appending the word "Material" and an integer i to the path to the file containing the material. Then, here, that name is changed by appending another integer, material_count, and the word "Robot". The problem is that the digits in i and material_name run together, so some combinations of i and material_count result in exactly the same string.

In the issue I linked to, the file package://moveit_resources_panda_description/meshes/visual/link6.dae contained (at least) 15 different materials, so AssimpLoader::loadMaterials named them:

package://moveit_resources_panda_description/meshes/visual/link6.daeMaterial0
package://moveit_resources_panda_description/meshes/visual/link6.daeMaterial1
...
package://moveit_resources_panda_description/meshes/visual/link6.daeMaterial14

Material14 was added when material_count=1120, resulting in the name
package://moveit_resources_panda_description/meshes/visual/link6.daeMaterial141120Robot

Later, Material1 was added when material_count=41120, which resulted in the same name, hence the ItemIdentityException. To fix this, I added an underscore before appending material_count to the material name, so for the preceding example we get two different names:
package://moveit_resources_panda_description/meshes/visual/link6.daeMaterial14_1120Robot
package://moveit_resources_panda_description/meshes/visual/link6.daeMaterial1_41120Robot

[jacobperron]

LGTM, thanks for the detailed explanation of the problem!

• Linux
• Linux-aarch64
• Windows

cpplint failures are unrelated.