R3BDataMonitor, R3BValueError and Update on FileSource2

[YanzhaoW]

Features

• Adding a new class R3BDataMonitor, a histogram manager, that could be used inside a task class.
• Adding a new class R3BValueError, a numeric data structure to represent physical values with errors. Popular operations, such as +, -, /, * are implemented.
• Update on R3BFileSource2 such that it can read a root file just containing a root tree. (todo: need a test)

This pull request is second part of the PR #890 .

---

Checklist:

• [x] Rebased against dev branch
• [x] My name is in the resp. CONTRIBUTORS/AUTHORS file
• [x] Followed the pull request guidlines and the Git workflow
• [x] Followed the seven rules of great commit messages

[YanzhaoW]

@jose-luis-rs Ok, this is ready to be merged.

[jose-luis-rs]

Thanks