add basic tutorial and update helpers

phase_diagram/LiAl_poscar

phase_diagram/export.csv

phase_diagram/helpers.py

phase_diagram/img/Picture1.png

phase_diagram/img/Picture2.png

phase_diagram/img/calphy_logo.png

phase_diagram/img/cimg1.png

phase_diagram/img/cimg2.png

phase_diagram/img/cimg3.png

phase_diagram/img/cimg4.png

phase_diagram/img/cimg5.png

phase_diagram/img/fig1.png

phase_diagram/img/logo.png

phase_diagram/img/phase_dia_1.png

phase_diagram/img/potentials_logo.png

phase_diagram/tutorial.ipynb