Bug fix for edge case in _compute_drho

[s-kuberski]

I finally got the indexing in the _compute_drho routine correct. My last fix did not solve all the edge cases that one can find and in one case, I ran into an error because the code tried to add arrays of non-equal size. I am fairly sure that the counting is now correct.

[fjosw]

Looks good, thanks!

[s-kuberski]

I tried to check this along the lines of

w_max = 49
r = range(100000)
for i in range(1, w_max):
    lll = [len(r[i + 1:w_max]),
        len(r[i - 1:None if i - w_max // 2 <= 0 else 2 * i - (2 * w_max)//2 : -1]) + len(r[1:max(1, w_max - 2 * i)]),
        len(r[1:w_max - i])]
    assert(np.all([o - lll[0] == 0 for o in lll]))

which did throw an error with the old counting.