Added option to remove mutants ABSENT in other file.

(Previously there was only an option to remove mutants present in
another file).

Added remove_mutants_not_in_other_dataset method to
Insertional_mutant_pool_dataset in mutant_analysis_classes.py; renamed
remove_mutants_based_on_other_dataset to remove_mutants_in_other_dataset
for clarity.

Changed some of the command-line option letters/words for
mutant_count_alignments.py and mutant_join_datasets.py, and added others
for the new option.

Added new mutant_count_alignments.py run-tests for this - they all pass.