DataTable fragment minor fixes

- NullHeaderFragment and NullFooterFragment deleted, NullFragment introduced
- added licence headers where missed
- methods which returns WebElement should be named with Element key word
DataTableWithFooterAndHeader removed

- it was difficult and unclear to create abstract classes for extended datatable, data table and collapsible data table
- now it is more clear
Metamer & Fragments: EDT fragment added, tests ported
EDT & DataTable added javadoc