add randomFraction argument to makeVariantDesign

Here's a summary of the changes:

1. Added the randomFraction argument to the function definition with a default value of 1.
2. If variant is 0, set randomFraction to 0.
3. Initialize dra and ddec arrays with zeros.
4. Generate random indices based on the randomFraction
5. Apply random offsets only to the selected indices in the dra and ddec arrays.