Fix PCR bug (issue #279).

[carreter]

Changes in this PR

In generatePcrFragments(), there was a loop that was erroneously iterating over the indices of forwardPrimerIndxs rather than the values of the slice itself.

Why are you making these changes?

Fixes #279 .

Are any changes breaking? (IMPORTANT)

No.

Pre-merge checklist

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ready for merging. Mergeable PRs will be prioritized for review.

• New packages/exported functions have docstrings.
• New/changed functionality is thoroughly tested.
• New/changed functionality has a function giving an example of its usage in the associated test file. See primers/primers_test.go for what this might look like.
• Changes are documented in CHANGELOG.md in the [Unreleased] section.
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