Don't offer to create indexes for remote files.
Further refinements of on-the-fly indexing.   All tribble supported files are now treated <almost> uniformly.  Small files that are not indexed are just loaded.   For larger local files, defined to be > 10mb,   an option is presented to creat an index.    An index is required for VCF files > 10mb.  The requirement for other file types is set at 1GB.
IGV-2061 -- feature visibility window cannot be set to zero (i.e. no window) on VariantTrack