changed the internal thread implementation to boost::thread and make use of the interruption points to properly terminate the application

use NDRegisterAccessor class instead of ProcessArray, so that e.g. the ConstantAccessor works as well

Fixed a bug that a pointer to a temporary VariableNetworkNode object was sometimes stored (as a trigger receiver or source). Instead now an object with the same data pointer inside is stored.

fixed a bug in the test: a module was defined at the same scope as the application, which lead to crashes when terminating the application sometimes
(same as last commit, but for a different test)

fixed a bug in the test: a module was defined at the same scope as the application, which lead to crashes when terminating the application sometimes

added function to obtain list of accessors from modules recursively (i.e. including accessors from sub-modules)

Added names and owners for modules, to build a full hierarchy chain. The syntax for creating modules is now not so nice, a better way has to be found (maybe by adding a macro if better solution can be found).

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

moved VariableNetworkNode member to AccessorBase class, so the network node information can be accessed also when having only the pointer to the base class

allow specifying the array length for direct connections between device and control system (as the type is already defined in this case)

- fixed copying only the first element in some situations

Added description field to the variables. It is currently ignored and will be added to the XML file in a next step.