generalise_gene_type: Summarise gene type
In mumichae/DASSIE: Detection of Alternative Transcription Start Sites

Add a column named general_gene_type to the data.table containing generalised gene type names

generalise_gene_type(
  dt,
  mapping = NULL,
  column = "gene_type",
  protein_coding = "protein_coding",
  general_column = "general_gene_type"
)

`dt`	data.table with at least a column containing specific gene types
`mapping`	mapping of specific to general gene type as data.table or list of vectors if kept NULL, then a basic mapping of pc (protein_coding) vs. npc is created
`column`	specifies the column of the specific gene type
`general_column`	name of general gene type column to add