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.

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

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.

improved structure of network creation and added a FanOut to distribute values to multiple consumers

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.