Raises a ValueError for a confliction between dimension names and level names

[fujiisoup]

• Closes Confusing behaviour with MultiIndex #2299
• Tests added
• Tests passed
• Fully documented, including whats-new.rst for all changes and api.rst for new API.

Now it raises an Error to assign new dimension with the name conflicting with an existing level name.
Therefore, it is not allowed

b = xr.Dataset(coords={'dim0': ['a', 'b'], 'dim1': [0, 1]})
b = b.stack(dim_stacked=['dim0', 'dim1'])

# This should raise an errors even though its length is consistent with `b['dim0']`
b['c'] = (('dim0',), [10, 11, 12, 13])

# This is OK
b['c'] = (('dim_stacked',), [10, 11, 12, 13])