Add function to infer shapelet order from number of coefficients
Pass coefficient array to ShapeletFunctor ctor as const

This should always have been a const array, as we copy it and hence never modify
the original.
Add FunctorKey for ShapeletFunction
Add FunctorKey for MultiShapeletFunction
Throw when we try to create a MultiShapeletFunctionKey to a nonexistent field.
Merge branch 'u/jbosch/DM-1224'