SiloFactory#data of type of actual "data" to be created in silo

[normenmueller]

It feels more natural to have data of type of the actual "data" to be created in a silo, rather than of type LocalSilo. The latter can be handled by the framework.

For example, let's have

class TestSiloFactory extends SiloFactory[Int, List[Int]] {
  def data: List[Int] = List(40, 30, 20)
}

rather than

class TestSiloFactory extends SiloFactory[Int, List[Int]] {
  def data = new LocalSilo(List(40, 30, 20))
}