making the code more python3 capable

    Making plt-> dst conversion a bit clearer
    new function plt.get_dst returns a dst object
    function plt.save_dst forces you to provide a file name (and use get_dst)

    "dic_cls" in LATTICE class updated according with the update of lib_tw_elem.py

    (1) ERROR_STAT/DYN_FILE command implemented. (2) The initial comment part updated.

    initial files for sphinx-doc

    fixed plt.save_dst

    print was not working correctly on both python2 and 3
    this has been tested to give correct result on both

    documentation/readme edits

    other minor fixes