Vcf improvements and fixes

[charlesgregory]

• Added Genotype and GT structs to aid in conversion of VCF "GT" format data to string.
  • Should also allow for modification as it is a slice to the backing format data
• Added ability to specify output type via VCFWriter
• Added multithreading support for VCFReader, VCFWriter, and TabixIndexedFile
• Fixes
  • VCFWriter ctor incorrect #116
  • FormatField can return strings with NULs #124
  • VCFWriter output modes #117

Let me know what you think and if there is anything else I should add in.

[codecov-commenter]

Codecov Report

Merging #125 (0dff527) into develop (2bfaf32) will decrease coverage by 0.45%.
The diff coverage is 67.24%.

@@             Coverage Diff             @@
##           develop     #125      +/-   ##
===========================================
- Coverage    85.85%   85.40%   -0.46%     
===========================================
  Files           43       43              
  Lines         3895     4151     +256     
===========================================
+ Hits          3344     3545     +201     
- Misses         551      606      +55     

Impacted Files	Coverage Δ
source/dhtslib/tabix.d	78.66% <53.84%> (-5.95%)	⬇️
source/dhtslib/vcf/reader.d	94.11% <60.00%> (-3.39%)	⬇️
source/dhtslib/vcf/record.d	87.56% <65.68%> (-3.93%)	⬇️
source/dhtslib/vcf/writer.d	93.71% <75.00%> (-6.29%)	⬇️
source/dhtslib/bed/reader.d	100.00% <100.00%> (ø)
source/dhtslib/gff/reader.d	96.61% <100.00%> (ø)
source/dhtslib/vcf/header.d	95.61% <100.00%> (+0.02%)	⬆️

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[jblachly]

LGTM