- add as.data.frame method for several 'berlin_data_*' classes. TODO: tests
- add is method for several 'berlin_data_*' classes. TODO: tests (partially done)
- add download method and examples for several 'berlin_data_*' classes. TODO: tests
- add subset method for 'berlin_data_*list' classes. TODO: tests
- remove XLS download support (it was illusory)
- add TXT download support. TODO: tests
- general code reorganization - core class methods in class files, miscellaneous methods (e.g. download.X) in other-methods.R