1) Make more obvious in back_substitute that outer
loop can be made parallel.
2) copy a_istk to my_istk instead of just refering
from my_istk.
3) Copy all of rhs to o_r by using newly code
space_outer member function of my_istk class. Then use same offset for o_r that's used for my_x in SOLVE_TRIDIAG_VERIFY code.