Intermediate commit.

Work on subcommit feature is not complete.

[ci skip]
Merge branch 'subcommands' of github.com:ulrikpedersen/swmr-testapp into subcommands
Intermediate commit
Turned SWMRWriter into a proper module

 * Removed main function
 * Moved class definition into a separate header file
 * Turned function args into const references where possible
Implemented the single application CLI with read and write subcommands
Travis: run the new single application