add strand flip check

[alkaZeltser]

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

For some background on strand flipping, check out my new discussion post:
#68

@RoniHaas I would appreciate your review of the key strand flip function (assess.pgs.vcf.allele.match)

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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• I have added the changes included in this pull request to NEWS under the next release version or unreleased, and updated the date.

• I have updated the version number in metadata.yaml and DESCRIPTION.

• Both R CMD build and R CMD check run successfully.

Testing Results

All unit tests pass

[alkaZeltser]

For those who are interested, check out the rendered manual:
ApplyPolygenicScore_2.0.0.pdf

And the rendered User Guide vignette:
UserGuide.pdf