libcerf function voigt_hwhm not suitable for external use

[Ethan A M. <me...@uw...>]

In response to a gnuplot feature request, I added a wrapper in
gnuplot to find the width of the Voight profile by calling libcerf function
voigt_hwhm().

However on further evaluation this has turned out to be problematic.
The current libcerf source for this function deals badly with out-of-range
input.  It prints to stderr (problematic by itself) and then either
- returns an incorrect value
- calls exit(-1)

A call to exit() from a library routine is almost never a good thing to do,
since it prevents error-handling and possible recovery by the calling
program.  Limiting error reporting to a message on stderr also prevents
detection and recovery by the calling program.

I suggest that it would be preferable to return NaN whenever the
input is out of range or the function does not converge.

Alternatively libcerf could provide an error-check routine or status variable,
but then you would run into issues of thread-safety and so on.

I am hoping that the libcerf function can be improved in a subsequent
version, but for now I will consider implementing equivalent code
directly in gnuplot.

	Ethan