allow helix index changing in batch

[dave-doty]

Allow both the index of each helix in the list of all helices to be altered, and also the view order specified in DNADesign.helices_view_order.

Furthermore, now that helices are not constrained to be consecutive (#172), allow to change helix indices to be any integers.

[dave-doty]

This was partially addressed by commit 2eab946, which allows changing the index of a helix one at a time. I'm leaving the issue open to allow a faster way to re-index all helices at once. Currently they can only be changed one at a time. Swapping two is particularly annoying since one needs to assign one of them a temporary index not belonging to any helix to avoid collision.

There's also currently no way in the web interface to leave indices alone but change the helices_view_order property of a HelixGroup. Update: This can now be done by adjusting the current HelixGroup:

[dave-doty]

Unlike helices_view_order, which has a meaning that does not depend on the old helices_view_order, re-numbering helices requires that you know the old helix numbers when assigning new ones. So entering it the same way as helices_view_order would be harder to use, since, after erasing the old list of indices, one would have to remember what order they appeared in for the new indices to make sense.

So, it seems best to implement this in a more structured way, where the hold indices are displayed read-only, with each of them next to a text field where the new index can be chosen.