Gauss3: ExecutorService can now be provided from outside

- Still there exists a way to pass numThreads
SeparableSymmetricConvolution: Using external ExecutorService now
LocalExtrema: Using external ExecutorService now
DogDetection: Using external ExecutorService now
DifferenceOfGaussian: Using external ExecutorService now
SeparableSymmConv.: Fixed JavaDoc
revert numTasks computation for SeparableSymmetricConvolution to the original formula, and make it more explicit in other places
fix javadoc.
undo formatting changes wrt to master and clean up
Merge pull request #67 from imglib/algorithm-executor-service

Algorithm Executor Service