Fiji Archipelago

Allows ElasticLayerAlignment to use a Fiji archipelago.Cluster.

Primary changes:
Use an ExecutorService to submit BlockMatchPMCC point matching
* https://github.com/larrylindsey/TrakEM2/blob/master/TrakEM2_/src/main/java/mpicbg/trakem2/align/ElasticLayerAlignment.java#L982
Addition of the ThreadPool and ExecutorProvider classes, to allow an ExecutorService to be supplied externally. This was done to allow the use of archipelago.Cluster without an explicit dependency on it.
* https://github.com/larrylindsey/TrakEM2/tree/master/TrakEM2_/src/main/java/mpicbg/trakem2/concurrent