- modify .delegate() to mirror .reduce()
- modify delegateByRange() to behave like delegateByFile()
- promote delegate* functions to generics
- remove 'byRange' slot in GenomicFileViews; add 'BY' arg to .delegate()
- add fileListElements generic and methods; goal is to extract a list
  form of file/index for bplapply
- change names of GenomicFileViews accessors to singular