Overview report variant table and IGV snapshots are not in sync

[stevekm]

Right now, the variant summary table displayed in the overview report is taken from the summary table TSV file, while the snapshots come from whatever files are found in the IGV_snapshots directory. This is not ideal because it is possible for issues to arise in the snapshot creation, which can lead to variants from the table not showing up in the report. Need to redesign the report code such that these aspects are built in parallel and include some checking to make sure all variants in the table have their associated snapshots.