material-specific logic is now removed from the material library and
encapsulated in the material class itself, and materials are loaded
from $FG_ROOT/materials.xml rather than $FG_ROOT/materials. This also
removes a nasty bug in the old material-loading code that caused a
floating-point exception.
pointing to a non-existant material name. (This has been fixed.)
The alias code didn't check to see if this condition would occur so it
was putting a 'NULL' entry in the material library which was causing
the crash. It now checks for and skips bad aliases and prints a
warning message.
