Backend function for retrieving files sequence and annotation files from the ENSEMBL ftp server
Arguments
- organism
- Organism selector id, there are three options to characterize an organism: - by - scientific name: e.g.- organism = "Homo sapiens"
- by - database specific accession identifier: e.g.- organism = "GCF_000001405.37"(= NCBI RefSeq identifier for- Homo sapiens)
- by - taxonomic identifier from NCBI Taxonomy: e.g.- organism = "9606"(= taxid of- Homo sapiens)
 
- type
- character, biological sequence type (e.g. "dna", "cds") 
- id.type
- a character, default "toplevel". id type of assembly, either "toplevel" or "primary_assembly" for genomes. Can be other strings, for non genome objects. 
- release
- a numeric, the database release version of ENSEMBL ( - db = "ensembl"). Default is- release = NULLmeaning that the most recent database version is used.- release = 75would for human would give the stable GRCh37 release in ensembl. Value must be > 46, since ensembl did not structure their data if the standard format before that.
- path
- location where file shall be stored. 
- format
- "gff3", alternative "gtf" for ensembl. 
