tri element module and isotropic material module

today have been working on these two modules, but unfinished.
need to further work on tri elem module, especially the integrate
procedure, the toolkit module, the global lib module
isotropic type module needs to be tested
test data hiding
ideas: use data hiding to protect objects from being randomly modified;
only its assoc. procedures in its native module can operate on it
make a exit_function procedure in parameter module, use it instead of
stop or call xit
in the global lib module, make sure to use save

tomorrow, finish the above and then quickly fill in the rest of elements
and materials