GenericMonitoringTool: set correct default value for HistPath

The default value for the HistPath property was by mistake changed in commit 4a2edffa and broke the automatic booking of histograms under the tool's parent's name. Revert the default value back to an empty string and improve documentation.

Also some general cleanup of the code:

• remove irrelevant virtual
• remove declaration of InterfaceID
• remove import of std namespace as std:: was used in most places
• spell and grammar check