autopaste: paste strand with translation auto-calculated from last paste

[dave-doty]

This idea originated with ENSnano, developed by Nicolas Schabanel and Nicolas Levy.

Currently, when pasting a copied strand, the only way to specify where it goes is to place the mouse cursor as a guide.

Frequently, it is the case that we will copy strands, paste them at a certain translation away from the original, and then want to repeatedly paste other copies at the same translation from the last pasted strand. For example, we copy these strands:

we manually paste them below:

Now, press Ctrl+Shift+V to autopaste. The strands will be pasted again, with the same translation (down by two helices) as between the original copied strands and the first paste:

Pressing Ctrl+Shift+V repeatedly will continue pasting strands along this same translation until they cannot be pasted anymore (because it would be out of bounds for the current helices):

More generally, the "translation vector" between the copied and pasted strands is in "y-axis units of Helix idx, in order of helices_view_order", "direction" (forward or reverse), and "x-axis units of offsets".

It is only possible to use this within one HelixGroup.

Combined with issue #579, this could reduce the time it takes to create a large design.

Even faster default translation

For even faster pasting for a common translation, if a strand has been copied, but no paste has occurred yet, then it is assumed that the translation is computed as follows

1. Move strands down (increase Helix idx, or more generally iterating through Helices in order of helices_view_order in the HelixGroup), keep forward the same, until first helix where they can be pasted legally.
2. If rule 1 is not possible with existing helices, move right (increasing offset, keep Helix idx and forward same) until they can be pasted.
3. Otherwise don't paste anything.

If strands come from more than one HelixGroup, don't paste.

For example, if we select the following strand:

and press Ctrl+Shift+V to autopaste (without having pasted already), this is the result:

This can be repeated as before: