Define the convention of the transformation matrix properly

[zhubonan]

The definition of the transformation matrix should be stated somewhere. It is different for row and column lattice matrices, here we use the column lattice matrix convention so

$$M c = C$$

Where c and C are matrices made of column lattice vectors.

This means for row vector matrices we have:

$$c^TM^T = C^T$$

Note： both ase and pymatgen stores the lattice as row vectors.

[zhubonan]

Notes added in #44 to highlight the difference caused by row and column vectors.