clustering: Throw exception when get_k_center/get_core_members is called before clustering is performed

[rimms]

clustering::get_nearest_center raises an exception when called before clustering is performed. (See. #150 and #95)

But, clustering::get_k_center and clustering::get_core_members does not raise an exception in the same situation. (empty list will be returned)

It's confusing.

I think clustering::get_k_center and clustering::get_core_members should raise an exception when called before clustering is performed.

[kmaehashi]

Fixed via #207