reimplement flipCell, fix https://answers.launchpad.net/yade/+question/701782

There were multiple problems in previous implementation. All fixed in 33e871fe (the first commit); the next commits are not changing the implementation in itself.

1. There are three main flips along X,Y,Z (the vectors of hSize). The total flip must be defined by recursive multiplication: flip=flipX * flipY * flipZ (each factor has determinant=1, leaving hSize.determinant unchanged). Instead, previous implementation would sum the terms, as in flip=flipX+flipY+flipZ, which opens up the possibility of a singular flip matrix like ((1,1,1),(1,1,1),(1,1,1)) (det=0, final volume=0).
2. We can't flip each coordinate independently. Suppose X is the axis being flipped, if we flip Xy first, then Xz, it is possible that the second flip invalidates the first one. Leaving the cell elongated.
3. When flip along X is decided, we need to determine the next flip (along Y) by considering a modified hSize (flipped along X). Instead, previous code would go back to virgin hSize for all three flips, possibly giving an irrelevant combination of flips (and, why not, a singular matrix and null volume).