The VoidAccessor comes along with the convenience classes VoidInput and VoidOutput,
and the TestFacility function getVoid()

* add HierarchyModifyingGroup

* VirtualModule: allow nested oneLevelUp etc.; simplify implementation

This is achieved by properly setting the _owner of each VirtualModule (unless highest level).

The simplification comes from then being able to use getOwner() instead of keeping track of "grandparents", and from using createAndGetSubmodule() instead of implementing something similar again in findTagAndAppendToModule().

* DataLossCounter constructor now takes qualified path to trigger variable

- ConfigReader is in ApplicationCore.h, but was including ApplicationCore.h itself
- Added include guards to ApplicationCore.h, although it's not stricktly needed (all included headers have one). Might speed up complilation because it makes it easier for the parser.

add ConfigReader.h to ApplicationCore.h, as the ConfigReader is now more strongly integrated with ApplicationCore

introduced a ModuleGroup (grouping ApplicationModules and other ModuleGroups) and enforced a proper hierarchy of modules to exclude that an ApplicationModule can own (directly or indirectly) another ApplicationModule

moved the xml generator main function into a new header file, to avoid defining main() multiple times by mistake.

added preliminary support for tags to application variables which can be searched for and used to generate virtual modules

Use mtca4u::setDMapFilePath() instead of the BackendFactory's member function. Since all applications will need it, the corresponding header file is now included in ApplicationCore.h

Introduced the DeviceModule class which provides an interface to obtain VariableNetworkNodes for device registers

Introduced the VariableNetwork describing the connections of variables/accessors before things get actually instantiated. This is needed to generate an XML file containing the published variables based on that information.