tr2g_GRanges: Get transcript and gene info from GRanges
In sarangian/deaRscripts: Differential Expression Analysis based on the read count data generated by either of Salmon, Kallisto, featureCounts and perform statistical analysis to discover quantitative changes in expression levels between two different experimental groups

Internal use, for GRanges from GTF files

tr2g_GRanges(gr, type_use = "exon", transcript_id = "transcript_id",
  gene_id = "gene_id", gene_name = "gene_name",
  transcript_version = "transcript_version",
  gene_version = "gene_version", version_sep = ".")

`gr`	A `GRanges` object. The metadata columns should be atomic vectors, not lists.
`type_use`	Character vector, the values taken by the `type` field in the GTF file that denote the desired transcripts. This can be "exon", "transcript", "mRNA", and etc.
`transcript_id`	Character vector of length 1. Tag in `attribute` field corresponding to transcript IDs. This argument must be supplied and cannot be `NA` or `NULL`. Will throw error if tag indicated in this argument does not exist.
`gene_id`	Character vector of length 1. Tag in `attribute` field corresponding to gene IDs. This argument must be supplied and cannot be `NA` or `NULL`. Note that this is different from gene symbols, which do not have to be unique. This can be Ensembl or Entrez IDs. However, if the gene symbols are in fact unique for each gene, you may supply the tag for human readable gene symbols to this argument. Will throw error if tag indicated in this argument does not exist.
`gene_name`	Character vector of length 1. Tag in `attribute` field corresponding to gene symbols. This argument can be `NA` or `NULL` if you are fine with non-human readable gene IDs and do not wish to extract human readable gene symbols.
`transcript_version`	Character vector of length 1. Tag in `attribute` field corresponding to transcript version number. If your GTF file does not include transcript version numbers, or if you do not wish to include the version number, then use `NULL` for this argument. To decide whether to include transcript version number, check whether version numbers are included in the `transcripts.txt` in the `kallisto` output directory. If that file includes version numbers, then trannscript version numbers must be included here as well. If that file does not include version numbers, then transcript version numbers must not be included here.
`gene_version`	Character vector of length 1. Tag in `attribute` field corresponding to gene version number. If your GTF file does not include gene version numbers, or if you do not wish to include the version number, then use `NULL` for this argument. Unlike transcript version number, it's up to you whether to include gene version number.
`version_sep`	Character to separate bewteen the main ID and the version number. Defaults to ".", as in Ensembl.