Add dynamic power node

Summary

Introduce a dynamic power node with runtime degree.

Major difference to other nodes are:

• No default constructor. Needs the size information as runtime argument.
• No CHILDREN static member. This cannot have a meaningful value
• degree() is non-static and returns a std::size_t
• Node is power-node in the sense of isPower == true: all children have the same type.
• Node is dynamic power-node in the sense of the DynamicPowerNodeTag. This has no direct (inheritance) relation to PowerNodeTag.

See dune-functions!285 (merged) for an example implementation of a dynamic power basis node in dune-functions.

Notes

• The implementation is extracted from !55 (closed)
• Requires !85 (merged) and !90 (merged) to be merged before