emodel: Started to implement children_slice_get
emodel: Added Slice event.
emodel: Start an impl. of slice get not keeping data in memory - it has drawbacks but I'll test it more and also will implement another version that does keep data allocate in memory during execution.
emodel: Added TODO comment about a possible issue.
emodel/emodel_test: This is a bigger commit, part 1/x(?) including some design modifications.
emodel: children_get and children_slice_get don't raise events any more. Using callback instead.