started an XSPECwiki page on low count spectra

I've started an XSPECwiki page on fitting low count spectra.

https://astrophysics.gsfc.nasa.gov/XSPECwiki/low_count_spectra

xselect failing to find event files

xselect gives a really misleading message when the event file cannot be found :
The input file is neither a FITS file nor an ASCII file
containing a list of FITS files
I've improved this to make it clear that the file is not being found.

xspec fix for DOS-mode scripts

12.4.0c DOS-mode scripts (with their extra carriage-return characters) are not properly handled for multi-line commands. Our thanks to Laura Brenneman for pointing this out. This also fixes another scripting bug in which a line with 'n' followed by blank spaces is wrongly interpreted as 'newpar' rather than 'no'. Report added on Dec 27, 2007.

This patch modifies headas/Xspec/src/XSUser/UserInterface/ execscript.cxx, XSUtil/Parse/ XSparse.cxx