Suggested updates to periodic.py

[cbporch]

Consolidates element information into a single tuple, update relative atomic mass/uncertainty values (current file is missing changes since 2016).

Also, alters the atomic_number function to accept lowercase atomic symbols.

References:

• https://iupac.org/standard-atomic-weights-of-14-chemical-elements-revised/
• https://iupac.org/standard-atomic-weight-of-lead-revised/

Edit: full disclusure, my Chemistry knowledge is middling to amateur at best, so the reasons for the failing tests are entirely opaque to me.

[bjodah]

@cbporch thank you for these updates! Much appreciated. The test failures are actually due to overly tight tolerances (it doesn't make sense that I have tighter tolerances in those tests than the uncertainty associated with the average molar mass of the natural abundance of the isotopes of the element). I pushed a commit locking the mass used in those tests, if they pass I'll merge.

Thanks again!