Correct handling of "D" in mask_left(), with new merge_softclipped_* functions

[ahstram]

Correct handling of "D" in mask_left(), with new merge_softclipped_deletions() and merge_softclipped_insertions() functions to ensure "S" operations are not immediately followed by "D", "N", "I" or "P"

This should address issue #3

Note: I did not consider whether corresponding merge_hardclipped_* functions are also necessary