Write global monitoring quantities to file

[ia267]

Currently enstrophy and div(u) are printed to stdout but not persisted. Write global scalar quantities (kinetic energy, dissipation rate, enstrophy, div(u) max/mean) to a monitoring.csv file each output step (similar to time_evol.dat in Incompact3d). Implement as a monitoring_t type in src/postprocess/monitoring.f90.