Added MARCH message importance level

We needed an additional message importance level for progress reports
and other very high frequency partial messages so that users can
enable/disable progress reports without affecting more traditional
kinds of messages.  We could have used a separate Facility for
progress reports, but (1) WARN and above levels wouldn't make much
sense and (2) we wanted to keep progress messages attached to
particular software layer's facilities.

The name MARCH was chosen because it's the same length as the
other levels except INFO and WARN, and progress bars can look like
they're marching across the screen.
Facilities::print doesn't show impset if it isn't set; documentation
Tweaks to Facilities::impset and Sawyer::initializeLibrary