embarrassing bugfix + test fixing + addition of default constructor test

My embarrassing PR to fix bugs...

Well the bugs are in _chemical_mixture.h_, I forgot to put in the path, and I swapped two files, it gave weird results...  And I also had a typo in the constructor, forgot the boolean.

Also I've added the default constructor in the test, fixed the test (funny stuffs, du to history I think, I let the why-don't-we-check-the-molar-mass-twice feature, we should always check the molar mass twice :)

And I simplified a little bit the reading step.

If no one says nothing, I'll merge it somewhere this week-end.