'Extend the current system into a new one.
Take the base and normal units of the current system to merge
them to the base and normal units given in argument.
If not provided, name and description are overriden by empty strings.'

def extend(self, base, dims=(), name='', description=''):

    base = (self._base_dims + tuple(base))
    dims = (self._dims + tuple(dims))
    return DimensionSystem(base, dims, name, description)
