TrigMuonHypoMT: Fix histogram paths and python cleanup

Change of histogram booking path:

• Use a common sub-directory for all histograms produced by the same HypoTool class.
• Set the name of the private monitoring tool to "MonTool" as we do not need unique names for private tools.

Python code cleanup:

• Make code flake8 compliant and introduce a unit test to keep it that way.
• Many print statements where converted to log.debug, so there will be much less printout now.

@sshaw, you may want to cross-check my changes. There were some uses of non-existent variables (e.g. "threshold") that I replaced with what I thought should be the correct variable name. Also feel free to adjust the logging level of some of the messages back to info if really needed.