_calculate_drho for large time separations

[fjosw]

This PR fixes _compute_drho for i>w_max // 2 and closes #156. Irrespective of this we should maybe think about how the gamma_method should deal with observables that have values like nan and inf.

[s-kuberski]

Great, thanks! This seems to resolve the first issue.

In principle we could skip the whole procedure if the mean value is nan or inf.

The second issue in #156 is not yet resolved. I think the point is that we only want to go up to w_max // gapsize in the loop, because we use the indexing [gapsize::gapsize] here. This issue shows up for the nan obs, because the loop does not end before the last index is reached.
Would you like to add this in your pr?

[fjosw]

Thanks for spotting this I think the last commit should cover this case. I also added your example to the tests.

[s-kuberski]

Very good, thanks!