Enable IGB to directly consume data from UCSC JSON API

The response for the https://api.genome.ucsc.edu/list/tracks?genome=galGal6 without any additional filter has few parent and children tracks in it, and the parent tracks doesn't have any data in the https://api.genome.ucsc.edu//getData/track?genome=galGal6&track=cons77wayViewelements&chrom=chr1&start=1&end=248956 API, it says in the response to query only for the children and the parent container doesn't have data. Attached a screenshot below. To avoid these tracks to show up in the available tracks list we can use this URL instead https://api.genome.ucsc.edu/list/tracks?genome=galGal6;trackLeavesOnly=1 which has the trackLeavesOnly=1 parameter added to it which filters out the parents and gives us only the children in the response.

Additional API examples:

• List all of the tracks available for the mm39 mouse genome:

  https://api.genome.ucsc.edu/list/tracks?genome=mm39;trackLeavesOnly=1

• Get the all_mrna track data for the mm39 mouse genome on chromosome 1 from 60380220 to 60429457:

  https://api.genome.ucsc.edu/getData/track?genome=mm39;track=all_mrna;chrom=chr1;start=60380220;end=60429457

• Get the genomic sequence for the specified region on chromosome 1 for the mm39 mouse genome:

  https://api.genome.ucsc.edu/getData/sequence?genome=mm39;chrom=chr1;start=60380220;end=60429457

Current UCSC DAS genomes: https://genome.ucsc.edu/cgi-bin/das/dsn
Current UCSC API genomes: https://api.genome.ucsc.edu/list/ucscGenomes

Using the hg38 human genome as an example:
DAS Types (available files for that genome): https://genome.ucsc.edu/cgi-bin/das/hg38/types
DAS Entry Points (chromosomes for that genome): https://genome.ucsc.edu/cgi-bin/das/hg38/entry_points

The queries below should produce equivalent output
DAS: https://genome.ucsc.edu/cgi-bin/das/hg38/dna?segment=M:1,16569
API: https://api.genome.ucsc.edu/getData/sequence?genome=hg38;chrom=chrM

Documentation

DAS: https://genome.ucsc.edu/FAQ/FAQdownloads.html#download23
API: https://genome.ucsc.edu/goldenPath/help/api.html