[HM] Initial total stress input

With the changes in this MR, the initial stress can be input as total stress. An optional attribute named total is added to the initial_stress tag to indicate whether the input initial stress is total stress or effective stress.

A benchmark is prepared to verify the implementation. The benchmark uses a [0,1 \text{m}]^3 cubic domain. The initial pore pressure is 10^6 Pa. the initial total stresses are \sigma_{xx}=\sigma_{yy}=-0.8\cdot 10^6 Pa, \sigma_{zz}=-2\cdot 10^6 Pa. The boundary conditions are given as

Boundary	H	M
top	p = 10^6 Pa	\tau_z = -2\cdot 10^6 Pa
bottom	p=10^6 Pa	u_z=0
others	-	u_n=0

With the Biot' constant being 0.8, the output effective stresses are expected as \sigma_{xx}=\sigma_{yy}=0 Pa, \sigma_{zz}=-1.2\cdot 10^6 Pa.

TODO: The same implementation for THM, RM, TRM, and TH2M.

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