build xacro files at installation time; rename resources folder to 'r…

[huaijiangzhu]

…esources'

Description

This PR adopted similar changes in https://github.com/open-dynamic-robot-initiative/robot_properties_bolt/commit/179ab2a9e8d5cad1cd4dde1d8c88108864c14cf3 and https://github.com/open-dynamic-robot-initiative/robot_properties_bolt/commit/aaa6b78fad781e508240fcceb327825260525978

These include:

• enabled building urdfs from xacro files at installation
• renamed the resources folder to resources
• removed utils.py and implemented resources.py to store the paths.

How I Tested

I installed the package using pip install . and ran the demos.

Do not merge before

• [ ] link to dependent pull request

I fulfilled the following requirements

• [ ] All new code is formatted according to our style guide (for C++ run clang-format, for Python, run flake8 and fix all warnings).
• [ ] All new functions/classes are documented and existing documentation is updated according to changes.
• [ ] No commented code from testing/debugging is kept (unless there is a good reason to keep it).

[huaijiangzhu]

@jviereck any comments? otherwise I will merge the pr.

[jviereck]

Looks good to me. As Max was more involved with the refactoring before I relied on him giving you feedback.

About making bullet_utils available on pip, that sounds like a good idea.