The improvement on the Burgers 1e3 ctest is around 0.96%.

Again auto keyword and Eigen do not always work together
as expected.

These values are only used inside the constitutive relation
computation and are not required to be saved for next
global iterations/time-steps.

Before this change the GM, eps_M_t, and eps_K_t values
were not initialized properly.

The loadState() function takes care of the latter two;
The values are extracted into own temporal structure.

according to Tom's comments.

    member initializer list

cause problem in multithreads

[FL] Removed mutable type specifier

[FL] Simplified some calculations and changed the documentation

according to Dima's comments.

[FL] Corrected a documentation for an argument of the member of getdValue