move ApplicationModule::appConfig() to ModuleImpl::appConfig() so it can also be used from ModuleGroup

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

Added preliminary support for bidirectional variables. There are still some known issues, e.g. concurrent updates can still lead to race conditions. This will be addressed soon.

Unfortunatly this currently breaks the ServerHistory module, as it relies on slashes in accessor names. The MicroDAQ module had the same issue but has already been fixed. This fix can be adopted to the ServerHistory module as well.

removed incorrect implementation of operator[] and operator() in ApplicationModule (and use the implementations of ModuleImpl instead)

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

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

Renamed Application::testableModeThreadName() into Application::threadName(), since this will be used for other purposes as well.

Changed Module::connectTo() to accept also temporary module objects, e.g. as returned by findTag(). As a consequence, the operators [] and () of the modules had to be changed to const.

Rebased the ScalarAccessor and ArrayAccessor classes onto the mtca4u::NDRegisterAccessorBridge class.
Known problems present: code compiles but tests do not run

The readNoNBlocking() to obtain the initial values is now run on all ApplicationModules (incl. VariableGroups) before starting the first module, to ensure the intial values are really processed first.

Read all application inputs once before starting the module threads, so the initial values are correctly present.

For this purpose, additional changes were necessary:
- Added support for constant values as feeders and consumers (just ignore the writes)
- Connect all unconnected application accessors to constants after defining the connections and before creating the implementations
- Renamed the user-overridden function "initialise()" into "defineConnections()", since more actions are now necessary during initialise().
- The connections are now made (i.e. the implementations are created) during the initialisation phase

- extended the trigger tests
- removed some debug output

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

Intermediate commit before the weekend: added options to add an external trigger to a network to handle poll and push properly. Currently it does not work properly.