Add seed() method for entities.

This method should have been introduced at the same time EntitySeed was
introduced in Dune, but was forgotten. Notice that we make use the
EntityPointer class for seeds for CpGrid, since they are quite compact.
Fix whitespace in Entity.hpp.
Made constructor call explicit.
Merge pull request #55 from atgeirr/master

Add seed() method (and fix whitespace) in Entity.hpp