ENH: update elemental data to IUPAC 2016

All masses now correspond to "Atomic weights of the elements 2013". For elements where the weights are given as a range (due to the precision in the masses of the isotopes being much higher than the precision in measuring their abundances), the "conventional value" is used. This means that some of them have fewer decimal places, but now are in line with IUPAC recommendations.

Names of the elements have all been updated to include recently approved names.