MAINT: Update years in a few copyright headers
ENH: Add ceildiv function (division with rounding up)
ENH: Initial skeleton implementation of node slicing

Slice node makes it possible to select a subset of plates from a node.
The slicing works similarly as basic slicing in NumPy: it supports
integers (positive or negative), Ellipsis (...), newaxis (None) and
slices (start:stop:step).
ENH: Implement messages for the Slice node.

Slice node makes it possible to use basic indexing/slicing for the
plates. The message to the child and the parent are now implemented in
addition to mask handling.
TST: Add tests for Slice node messages.

Test constructor and the messages to children and the parent.