In collective/tree/tree.h Rename Tree::getNumTotalChildren to Tree::getNumDescendants

[PhilMiller]

Describe the bug

Recursive descendants of a node in a tree are not its children. Rename accordingly.

To Reproduce
Steps to reproduce the behavior:

1. Example/test/snippet of code that fails
2. Compiler, platform, libraries
3. Run command: number of processors, threading options, etc.
4. See error

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
If applicable, add screenshots to help explain your problem.

Platform (please complete the following information):

• Supercomputer or desktop [e.g., Trinity, Sierra)
• Compiler environment [e.g., clang-3.9, gcc8]
• Libraries [e.g., mpich-2, openmpi)
• Commit ID or release of VT [e.g., vt-1.0.0]

Additional context
Add any other context about the problem here.