Debug output for local and global matrices

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behavior change was documented? -> TODO follow-up MR

Features

• Output of local matrices can be requested with the following environment variables (both have to be set):
  • OGS_LOCAL_MAT_OUT_PREFIX="<some prefix>"
  • OGS_LOCAL_MAT_OUT_ELEMENTS="0 1 5 16 123" (or "*")
  • should work for all processes
• Output of global matrices can be requested via OGS_GLOBAL_MAT_OUT_PREFIX="some prefix>"
  • only implemented exemplarily for the small deformation process.
  • will be available out of the box for all processes supporting OpenMP assembly, in the future.

Possible local matrix output:

...

## t = 1, process id = 0, element id = 3

# b
-4.99844e-20
-2.42866e-20
 4.93996e-19
-4.19725e-19
  0.00274725
  0.00274725
 -0.00274725
 -0.00274725

# Jac
  0.576923  -0.384615  -0.288462  0.0961538   0.240385  0.0480769  -0.240385 -0.0480769
 -0.384615   0.576923  0.0961538  -0.288462 -0.0480769  -0.240385  0.0480769   0.240385
 -0.288462  0.0961538   0.576923  -0.384615  -0.240385 -0.0480769   0.240385  0.0480769
 0.0961538  -0.288462  -0.384615   0.576923  0.0480769   0.240385 -0.0480769  -0.240385
  0.240385 -0.0480769  -0.240385  0.0480769   0.576923  0.0961538  -0.288462  -0.384615
 0.0480769  -0.240385 -0.0480769   0.240385  0.0961538   0.576923  -0.384615  -0.288462
 -0.240385  0.0480769   0.240385 -0.0480769  -0.288462  -0.384615   0.576923  0.0961538
-0.0480769   0.240385  0.0480769  -0.240385  -0.384615  -0.288462  0.0961538   0.576923

...