[DIAGNO] add an option to write a more accurate output file for the afield_points diagnostic

[jonathanschilling]

[Similar as #234, but this time for the new afield_points diagnostic. Started working on that separately and wanted to wait for feedback on 234, thus only submitting now.]

With this patch, the user can use a DIAGNO_IN namelist flag lapoints_accurate_output to activate a different output file format for the diagno_atest.<runId> output file, which has:

• the number of points that were written in the first line (for easier parsing, similar to the fluxloop and segmented Rogowski output files) and
• more digits of accuracy for each of the outputs (the output format was copied from the corresponding writing routine for the segmented Rogowski sensors)

By default, this option is disabled in order to retain backwards compatibility.

Also, added writing of the flags lapoints_accurate_output and lbpoints_accurate_output to write_diagno_input.

[lazersos]

Looks fine to me.