Add option to output subdomain matrices in elliptic solver

[nilsvu]

Proposed changes

Similar to #5540 but outputs the subdomain matrix from each element. Here's an example for a Poisson problem on a Sphere domain with 2 shells (first two rows) and an inner cube (last row):

Upgrade instructions

In input files for elliptic executable, change the option ExplicitInverse to:

ExplicitInverse:
  WriteMatrixToFile: None

You can set this option to a filename to output the subdomain matrices for debugging or analysis.

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments