Added thermal conduction test to test file.  This is similar to the diffusion one, it uses OpenPNM as a resistor network and find T in each pore, then compares to an analytical solution.  The T difference should always be less than 0.015 K.
Added 2 shortcuts to Core for getting ALL pores or ALL throats using pn.Ps and pn.Ts.  These are @property methods similar to pn.Np and pn.Nt.  I was getting tired of typing pn.pores() and pn.throats() all the time.