TST: Added a test to compare the implemented CODATA definitions to those from scipy

I am not 100% sure if this is one is really needed. This commit contains a test for the new units module in which I implemented support for various CODATA versions some time ago. It checks whether the defined values actually match those that scipy contains. This test is not really necessary for the user, but may become helpful when implementing the next CODATA version. So, I leave it up to you to decide whether we need include it in master or not.