test_tri_elem and output module

- test_tri_elem started (unfinished)
- output module started (unfinished)
- rearranged the included files : lib_mat and lib_node nolonger included
under lib_elem, they are also directly included under main program
because their initialize routines are needed in the main program
- allocatable array is used in lib_mat for different material type
arrays

- a generic element type may be needed for the storage of global element
info (index, type, typekey, etc.), just like in the case of material
type; then allocatable arrays can be used in lib_elem for different
element type arrays