two letters atoms problem correction

[maillardj]

1. Modification in database.py to prevent failure when molar mass is unknown + test
2. Modification in modeldata.py to be able to recognize two-letters atoms + test on Ar

[codecov[bot]]

Codecov Report

Merging #15 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #15   +/-   ##
=======================================
  Coverage   23.93%   23.93%           
=======================================
  Files          19       19           
  Lines        1780     1780           
=======================================
  Hits          426      426           
  Misses       1354     1354           

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[erwanp]

Hello ! 2 small Git problems happened :

• in your second-to-last commit you removed lines from the test files, but they are still in Git history. So the repository size will not change.

• you Merged (= pull changes and create merge commit) instead of Rebasing (pull changes without creating a new commit), so now your own pull request includes changes from other people (see the list of commits above).

It will be quite hard to clean the history. What I suggest instead :

• save your changed and new files locally

• clone again, or reset to the latest version (04e4ffe)

• paste your changed and new files

• commit them in one or two commits (one for the changes, one for the tests?)

• open a new PR

[erwanp]

Fixed in #16