#178 moved the preparation of the retrieval from RetrievalListener.retrievalStarted() to SlidableWMSServiceLayerGroup.retrieve()
#178 changed the class of progressTable and layerComplete to ConcurrentHashMap and AtomicInteger
#178 use a local variable to calculate the progress instead of the field progress in RetrievalListener.retrievalProgress()
Merge branch 'bug/178-ConcurrentModificationException-in-SlidableWMSServiceLayerGroup' into dev