`renameaxis!` is not inplace anymore

[felixcremer]

Since the switch to DimensionalData the renameaxis! function is not inplace anymore and you need to reassign the cube to keep the changes. We can either get rid of the bang and update the docstring or we can try to make it actually replace the dimension again. I think, changing the docs is fine and maybe we should deprecate that function for funtionality from DimensionalData anyway.