Add convenience methods to Stranded

[ielis]

@julesjacobsen I propose adding default methods toPositiveStrand() and toNegativeStrand() to Stranded.

These methods would allow us to write a code that is easier to read. E.g.:

Variant variant = ...
GenomicRegion region = variant.toPositiveStrand().toZeroBased();

instead of current

Variant variant = ...
GenomicRegion region = variant.withStrand(Strand.POSITIVE).toZeroBased();

Please feel free to merge the PR if you agree.