- Add switchable SO_compliant behaviour to recognition of feature types.

- Add switchable "parse only" behaviour, useful for just parsing huge files
  without having to create the attendant data structs.
- Add switchable "default to basic feature" behaviour which makes a basic
  feature out of anything it doesn't recognise.
- change zmapGFFGetFeatures() to return a GData list of feature arrays.
- improve error diagnostics.