✨ Feature Request

As raised by @michaelsanderson42: CubeList.copy() should return a CubeList instead of a list.

Motivation

The unspoken contract of a .copy() method is that it returns a new object of the same type.

Additional context

CubeList is currently inheriting its .copy() behaviour from the list parent class. Docs link

import iris
a = iris.cube.CubeList()
b = a
c = a.copy()
type(a)
# <class 'iris.cube.CubeList'>
type(b)
# <class 'iris.cube.CubeList'>
type(c)
# <class 'list'>