portable matfile exporter (in C)

Sometimes it is convenient to export data as a binary matfile, so that it can be manipulated in Matlab. So here are two convenience functions to write matrices and strings. They don't rely on any Matlab libraries being available.

If your floating point unit is not ieee754 then the bits cannot simply be written out, so a portable floating point save routine has been provided. It handles NaNs, infinity, and denormalized numbers. So you can export mat files directly from a supercomputer running weird hardware, and you can write ANSI C programs knowing they will work correctly on every ANSI C compiler.