New treelib (1.7) mess up the print_hierarchy output

[daico007]

Bug summary

With the new treelib v1.7, the Compound.print_hierarchy is giving out different output (comparing to the last version v1.6.4), which is a bit off putting. We need to update the method to keep up with the new change.

Code to reproduce the behavior
v1.6.4 (expected)

v1.7.0

Software versions

• Which version of mBuild are you using? (python -c "import mbuild as mb; print(mb.__version__)")
• Which version of Python (python --version)?
• Which operating system?