Substantial rewrite of FGNewMat, the material class. Most of the

Substantial rewrite of FGNewMat, the material class.  Most of the
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.