Major refactoring
Major refactoring

- Remove Reads class
- Cleanup Identifier class
- FastQ class has now a select_rand_om_reads method. Fixed the split() method
- Remove FastQMerger class (was not implemented)
- FastQC is now based on FastxFile class to iterate through qualities and
  sequences
- update all docstrings and tests

sequana/sequana,sequana/sequana,sequana/sequana,sequana/sequana,sequana/sequana