Update `domain.py’

[keiyamamo]

Resubmit of #63

I tested against

1. Single fluid problem https://github.com/KVSlab/turtleFSI/blob/master/turtleFSI/problems/TaylorGreen2D.py
2. Single solid problem https://github.com/KVSlab/turtleFSI/blob/master/turtleFSI/problems/TF_csm.py
3. Two solid region problem https://github.com/KVSlab/turtleFSI/blob/master/turtleFSI/problems/Test_Cylinder/problem_aneu_2solid.py
4. Two fluid region problem https://github.com/KVSlab/turtleFSI/blob/master/turtleFSI/problems/Test_Cylinder/problem_aneu_2fluid.py
5. Turek flag
6. Robin boundary condition problem https://github.com/KVSlab/turtleFSI/blob/master/turtleFSI/problems/RobinBC_validation.py
7. Mooney-Rivlin material model problem https://github.com/KVSlab/turtleFSI/blob/master/turtleFSI/problems/Test_Material/UniaxialTensionMooneyRivlin.py

[keiyamamo]

Hi @dbruneau-mie

It is essentially the same pull request but I tested against the problems as you suggested. They all worked and seems safe to merge. Let me know if you have any opinions!

Best,