Output global monitoring quantities to a csv file

[ia267]

This PR adds global monitoring output to a csv file so that some key quantities can be tracked during the simulation.

Change

• add a new m_monitoring module that computes and writes:
  • time
  • enstrophy
  • maximum divergence
  • mean divergence
• write these quantities to monitoring.csv on output steps (it will still print to stdout)
• move the monitoring lifecycle into the base case so setup/finalisation is handled centrally
• replace the per-case enstrophy/divergence print helpers in the case implementations with the shared monitoring writer
• add user documentation for the new csv output, including column descriptions and a simple plotting example
• move postprocess.f90 into src/postprocess/ and update CMake accordingly

Closes #295