Closes #309: account for helix group rotation properly in oxdnaoxview export

[DanielHader]

Fixed Issue #309: account for helix group rotation properly in oxdna/oxview export

Incorrect rotation was caused by adding the helix position within its group to the helix group's position to determine helix position.

The helix's local position within its group must be rotated according to yaw, roll, and pitch of the group itself before being added to the group's position.

Additionally, the use of design.roll_of_helix(helix) instead of using the roll attribute of the helix group when calculating the oxDNA vectors necessary for export, is not the right thing to do in this function.

design.roll_of_helix(helix) adds the helix's local roll to the global roll of the helix group

• The roll of a helix group is given with respect to the origin of the helix group and represents the orientation of the entire group of helices
• The local roll of a helix is given with respect to the helix's center
  These don't line up unless the helix is at the origin of its helix group

It's not entirely clear that the roll_of_helix function of a Design even has a valid use-case

After the changes, the design is exported properly

Related Issue

Issue #309