Generic fld_output, global output_control and simcomp file output

[timofeymukha]

This PR introduces:

• output_control: "global" for simcomps, which syncs output_control and output_value with the fluid. Useful to control output of multiple thing by a single set of parameters.
• fld_output_t, which is much luck fluid_output but does not add the fluid fields to the output. Can be used to output any number of arrays to a new fld.
• Working on outputing simcomp fields to files. What I have now is that if you don't specify output_filename, the field (e.g. vorticity) gets added to the fluid_output in the case. Otherwise, a new fld file is created. Opinions welcome.
• A small refactor to the output_t constructor, so that one can use init in descendents as is now standard.

When working on this I realized that e.g. the probes simcom just manages its own output, without adding an output to the sampler. I'm not sure what is best and how things will go forwards, so here I focused on making it easier to use the sampler by adding fld_output_t.

[timofeymukha]

TODO: add the same to lambda2.

[njansson]

🍻

[timofeymukha]

@MartinKarp Whenever you have time :-).