Updated double precision constant divisions in helix.cpp

[Arpan3323]

Hi, I noticed some double precision constant divisions in helix.cpp and saw room for potential efficiency improvement as divisions are generally more expensive.

What is changed?

• Updated multiple occurrences of division by a constant double to multiplication by a constant double
• The updates are of this nature: some_double_precision_number / 2. to some_double_precision_number * 0.5

Remarks

I think compilers might optimize most of these divisions anyways for optimized builds. Feel free to close this pull request if these updates are not desired. Let me know if you have any questions or suggestions.

[biochem-fan]

Thanks. Probably the speed difference in practice is almost zero but I merged this anyway.