filter non-defined elementary flows

[mfastudillo]

Hi!

This is a pull request related to the issue identified here where the calculation crashes if we use samples of characterisation factors of elementary flows that are not present in the database.

I think this would fix the problem, I hope it does not break anything. It would be cool to have in the readme some instructions about how to run the tests :)

[PascalLesage]

Hi @mfastudillo, and thanks!
Unfortunately, the proposed strategy for filtering out the characterization factors for elementary flows not in the biosphere matrix does not work: https://ci.appveyor.com/project/PascalLesage/presamples/builds/35677211/job/s43vg7w6pb3nnesr
You can always check the test results directly on github when you commit new changes.
Also, you should include new tests to make sure your filter is working.
Looking forward to reviewing the next iteration.