Using Eigen matrix block to replace matrix reduce & increase

[DrChiZhang]

@Shuaihao-Zhang @Xiangyu-Hu
Shuaihao, in your present work of continuum dynamics, function of reduceTensor and increaseTensor is introduced.
Would please check the eigen matrix block feature of
https://eigen.tuxfamily.org/dox/group__TutorialBlockOperations.html
to simply the corresponding code?

[Shuaihao-Zhang]

@ChiZhangatTUM
Thanks! I think this is a better solution.
I will update the code.

[DrChiZhang]

Done.