Final concentrations are now in better agreement with required values.
For this, I modified the way concentrations are calculated. Troncated reals become rounded values, closer to required values.
This in turn induce a need for a carefull check of final number of atoms not to be superior to the number of possible atoms in the supercell.
The way single molecule is dealt with is also deeply changed.
There is now two way of having one single molecule.
First, you calculated the mole fraction that corresponds to 1 molecule in the supercell, now that user can chose the total number of possible molecule
per direction. If it is 5, the mole fraction that corresponds to 1 molecule per supercell is 1/5^3.
Second, you can let 0 as mole fraction. The program understand this as a wish to recalculate the other mole fractions so that they adapt to
a single molecule appearing in the supercell. In other words, the other mole fractions are slighlty changed. For instance, for 3 molecules of
composition 0.5 0.5 and 0.0 and 5 molecule per cell, then the real mole fraction of molecule 1 and 2 is 0.5-1/(5^3*2). The factor 2 in the denominator
is here because we distribute equaly the variation of mole fraction on all mole fractions different from 0. Note that there may be
several molecule with mole fraction 0.