getIsoPartsList: Parse isoform parts list from annotation
In fchen365/surf: Statistical Utility for RBP (RNA binding protein) Functions

Parse the isoform parts list from annotation.

getIsoPartsList(
  anno,
  gene_id = getMultiTxGene(anno),
  depth.intron = 300,
  cores = max(1, detectCores() - 2)
)

`anno`	a `GRanges` object of genome annotation, return by import.
`gene_id`	`character`, gene_id"s to analyze, default to all multi-transcript genes.
`depth.intron`	`integer`, extended depth into gene's flanks, default to 300 nt.
`cores`	`integer`, number of computing cores to use.

a list named by gene_id, each of which contains:

`segment`	`GRanges`, genomic data of parts list.
`label`	`integer` vector, gene model (0 for intron) and exon numbers (coded as integer 1,2,...).
`layout`	`lgCMatrix`, transcript structure.