add strand flip check

[alkaZeltser]

This PR adds allele match checking and strand flip correction functionality.

Closes #63

• Add DNA flip function (flips alleles to reverse complement)
• Add allele match check function (checks for various mismatch cases including strand flip, and corrects strand flips)
• Incorporate strand flip checking into the main apply.polygenic.score function
• Add tests for above
• Update all documentation (README, manual, vignette)
• Export all relevant new functions through NAMESPACE
• Update NEWS with new feature

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Testing Results

All unit tests pass