BOUT++ v4.2.3

This is a bugfix release:

• Add counters to Timers to avoid double-counting sections #1817 (ZedThree)
• Bugfix: cyclic parallel derivative inversion done in wrong space #1807 (ZedThree)
• Add instructions for Ubuntu 16.04 #1690 (dschwoerer)
• Allow relative paths to be followed when making #1670 (johnomotani)
• Update examples/staggered_grid for v4.2 #1645 (johnomotani)
• Add EXTRA_LIBS to LIBS when searching for libraries #1632 (ZedThree)
• Recent fixes/improvements for Travis into master #1627 (ZedThree)
• Bug in calculation of ShiftAngle (qloop) #1626 (bendudson)

See CHANGELOG.md for more details

Note: if you download the source code tarball from GitHub, please download the file BOUT++-v4.2.3.tar.gz rather than the GitHub generated tarballs. This is because the GitHub tarball does not include the submodules.