Added sorting of header tags.

Added metrics output for single-end input data.

Updated version nr.

Added merging of headers when using the sfm command with a directory

containing multiple files as input.
The merging respects the requirements outlined in the SAM spec:
- order of sequence dictionaries is kept
- read group identifiers are made unique if needed + optional rg tags of
  reads are updated accordingly
- program line identifiers are made unique if needed + optional pg tags
  of reads are updated accordingly
- comment lines are merged
- the sorting order is set to Unknown
If elprep cannot merge while guaranteeing the above constraints, an
error is produced.

Updated version number.

Updated go.mod file.

Removed elPrep 5 announcement.

Ensure that reads that are already marked as duplicate are handled co…

…rrectly in mark duplicates / mark optical duplicates.

Updated go.mod.

Various bug fixes and extensions:

* Added --tmp-path option for sfm.
* Fixed /dev/stdin and /dev/stdout for sfm.
* Added a command for merging intermediate metrics files.
* Fixed detection of directory path names in sfm.