[Feature request] Add isotopes mass and relative abundances

[RGB-es]

In the properties table for each element it says how many stable / unstable isotopes there are. It will be really useful to have instead the masses of the most important ones and their relative abundances.

I found the following resources:

https://www.nist.gov/pml/atomic-weights-and-isotopic-compositions-relative-atomic-masses

In that page it's possible to get isotope tables for all elements in several file formats (html, txt, etc.). The abundances ("isotopic composition") are normalized to 1.

Also:

http://moltensalt.org/references/static/downloads/pdf/stable-isotopes.pdf

https://www.sisweb.com/referenc/source/exactmas.htm

Many thanks for this incredible useful tool!

[petrkle]

Pull requests are welcome.