Synchronized writers

In Orfeo ToolBox, there are several filters that produces multiple output images. For now, writing all those output to separate files is inneficient, since the streaming mechanism will end up executing the filter on the whole dataset several time.

The workaround is to pack all those outputs in a single output image using ImageListToVectorImageFilter, but this is not a good solution in many cases : the different ouptut might represent very different things that should not resid in a single VectorImage, and sometimes the different ouptuts have different types (for instance if their is an output mask along with a scalar image).

We need to enable synchronization between writer, so that once a stream is calculated by the filter, all corresponding output files are updated.