Add funcionality to interate forward in a DwC-A file

Make the DwC-A iterator move fordward by chunks of size n
Change get_records to receive only the size of the chunk and not "start and limit" arguments
Update tests to respect new mofications to the next method and the inner working of the library
Fix error on correlation with the extensions
Improve test functionality by adding a run_cases function
Move parser and iterator variables to be class members