dlthx fortran-to-python error

[ab3ap]

There appears to be an indexing error in dlthx.py as compared to the fortran version in itm.for. However, test_dlthx.py passes as-is and also with the correction. The current dlthx.py line 65 is

     `dlthx1 = qtile(s[2:-1], ka-1) - qtile(s[2:-1], kb-2)`

but in fortran, lines 786-791, it seems the equivalent would be:

    `dlthx1 = qtile(s[2:], ka-1) - qtile(s[2:], kb-1)`