OpenFoam: warn about writing output data to Lustre

[willfurnass]

• OpenFoam can generate a lot of output files, all of which are by default small text files
• Lustre copes badly with lots of small files
• Users should try to ensure they create only a limited number of files in total and a limited number of files per directory. This could be through constructing OpenFoam simulations that only output data every e.g. 10,000 time steps
• Users should verify that the options for only outputting files every X timesteps are definitely working; this can be done by submitting some very short jobs.
• Users should get in touch if they have any questions

See http://www.archer.ac.uk/documentation/software/openfoam/ for more info/guidance/warning on using OpenFoam against Lustre.