Overwork inner and outer const correctnes of the assign operator of data storage

[SimeonEhrig]

During the work on PR #469 we realize that we do not cover all cases of assignment of DataStorages to DataStorages with different inner and outer const typs. We also not clarified which should be allowed and what not. This is the current state what is allowed, disallowed and unclear:

assignment: to = from

total 16 cases

4 cases:

to	from	allowed	comment
DataStorage<int>	DataStorage<int>	Y
DataStorage<int const>	DataStorage<int>	Y	const view of mutable source -> hidden modification possible
DataStorage<int>	DataStorage<int const>	X	Allows to modify inner const source
DataStorage<int const>	DataStorage<int const>	Y

8 cases:

to	from	allowed	comment
DataStorage<*> const	*	X	general not allowed to reassign a const object (C++ rule)

4 cases:

to	from	allowed	comment
DataStorage<int>	DataStorage<int> const	X
DataStorage<int const>	DataStorage<int> const	Y
DataStorage<int>	DataStorage<int const> const	X
DataStorage<int const>	DataStorage<int const> const	Y