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