Updated RNA model, fixed helix direction vector

[Lucandia]

The extendStrand method in the RNA model calculates the helix axis and generates an RNA helix starting from a nucleotide. Previously, the calculation of the helix axis had a minor inaccuracy, resulting in a noticeable kink when extending the RNA helix. This discrepancy caused the helix axis of the extended part to deviate slightly from the original helix axis. For instance (see the image), a 100 bp RNA helix was extended twice, making the kink particularly evident in the orthogonal view. I have refined the calculation of the helix axis based on the nucleotide coordinates. With this update, the extended RNA helix maintains a consistent axis, eliminating the kink. The RNA helix extension now appears smooth.