Variant position --> [start, end)

In an attempt to coalesce the variant models across several different systems, I would like to recommend using start, end instead of position.  Please accept my apologies if this has already been discussed.

Source  | Coordinate system | Interval type | Position fields | Contig names
------------- | ------------- | ------------ | ----------- | ------------
ADAM|0-based|closed-open interval|start, end|"1" or "chr1"
Ensembl REST client | 1-based | closed interval | start, end | "1"
GEMINI|0-based|closed-open interval|start, end|"chr1"
Global Alliance/GA4GH v0.5|0-based|?|position|"1" or "chr1"
Google Genomics/GA4GH v0.1|1-based|NA|position|"1" or "chr1"
VCF file|1-based|NA|position|"1" or "chr1"

I will also attempt pull requests at ADAM and Google Genomics for position --> start, end.

See also
https://github.com/bigdatagenomics/bdg-formats/pull/14
https://github.com/bigdatagenomics/bdg-formats/pull/13