Merge pull request #75 from Yonghui56/TH2C

Th2 c
abstract EOS class.
small changes.
get_n() function needs to be public.
1) derive from abstract class.
2) initialize dof in the constructor.
3) use get_n() function everywhere.
adding documentation.
this i is not used.
realize the abstract functions.
these are not needed, supposed to be passed from outside.
set up the abstract function set_env_condition()
now seperating two different EOS.
now N depends on the different EOS.
solving the right EOS system.
1) activate the local problem solution.
2) clean unnecessary functions.
Merge pull request #6 from HaibingShao/TH2C

Th2 c