Iterators in IndexedComponent

[bknueven]

In making Egret fully Pyomo 6 compatible grid-parity-exchange/Egret/pull/232, I noticed an incongruity and potential performance issue in IndexedComponent:

pyomo/pyomo/core/base/indexed_component.py

Lines 361 to 371 in acfe565

	def keys(self):
	"""Return an iterator of the keys in the dictionary"""
	return [ x for x in self ]
	def values(self):
	"""Return an iterator of the component data objects in the dictionary"""
	return [ self[x] for x in self ]
	def items(self):
	"""Return an iterator of (index,data) tuples from the dictionary"""
	return [ (x, self[x]) for x in self ]

These standard dict-like iterator methods all return a list, which is not as promised in their docstrings.