Constants (as generated by ApplicationModule::constant()) were causing a
logic_error if accessors of different types were using the same constant
(created with the same value and the same value type upon creation).
This is possible since the type passed to constant() does not have to be
the same as the type of the accessor.

This is now fixed by giving each constant a fully unique name, so every
accessor using a constant will be in its own little variable network.

- ControlSystemModule
- VirtualModule
- findTag
- excludeTag

and some related functions.

- Implemented but not tested yet
- All existing tests passing

* 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

Print path to the nonvirtual module if oneLevelUp or oneUpAndHide are
detected directly below the applicaiton root.

- Initial values are sometimetimes only propagated in the first run of the main loops, not in prepare. If other modules are waiting for it they cannot enter their main loop. That's why all threads have to be started first, and only the it is waited for all of them to enter testable mode.

Fixes #68

Implemented first part of #17: A HierarchyModifier can now be specified instead of the eliminateHierarchy boolean flag. No new functionality is yet added.

Use one file per class. Also, use boost::core::demangle instead of
checking for abi::__cxa_demangle by ourselves.

We have a visitor now for generating
 - the XML
 - the Module's dot graph
 - the textual dump of the VariableNetwork
 - the dot graph of the VariableNetwork

Proper implementation of move constructors and move assignment operators for all module types (including tests for the assignment operator)

fixed connectTo() so it now uses the dynamic/virtual model and thus obeys e.g. eliminated hierarchies etc.