COMP: Fix warning for overloading AllocateElements

Also remove the const qualifier for the bool argument for
AllocateElements. It is unexpected and not common convention to add a
const qualifier to arguments passed by value.

This patch addressed the following warning:
warning: 'TElement* itk::ImportImageContainer<TElementIdentifier,
TElement>::AllocateElements(itk::ImportImageContainer<TElementIdentifier,
TElement>::ElementIdentifier, bool) const [with TElementIdentifier =
long unsigned int; TElement = short int;
itk::ImportImageContainer<TElementIdentifier,
TElement>::ElementIdentifier = long unsigned int]' was hidden

Change-Id: I8c847ad9eaa9aa1a0cbf41169cde1ce0b67edbc7
Merge topic 'FixFactoryTestLibOverload'

6a28ed19 COMP: Fix warning for overloading AllocateElements