Improve formatting of error locations (Here() functions)

The key is to override the TObject::DoError method. The new version
functions without any calls to the static Form method, hopefully improving
thread safety as well.
Remove more instances of non-standard identifier __FUNCTION__
Remove support for defining global variables via obsolete TagDef interface

Also, use simplified THaAnalysisObject::LoadDB member function interface
wherever possible.

This is the first of several steps toward converting to a unified
database API.