new generic function reader

- implementation incomplete: methods must be called reader.dataformat.xxx
- support for gzipped an bzipped files is still missing
- tests are missing
rename generics to reader.dataformat.xxx, remove extension dispatch table

- Tests are still failing
new parameter override_extension to dataformat constructor; extract only the last part of a multi-part extension such as .csv.bz2
fix tests for .file format

- all tests pass, but these are still mostly false positives