SF: Support meta data from operation to use custom marker symbol in plotter

[MichaelHuth]

For some single data point results from operations like powerspectrum we could use an additional information transported through a meta data field that allows the plotter to change the marker shape in the plot. For example if a the powerspectrum determines a ratio and the fit failed.

[t-b]

This should tie in properly with #1525 when/if tackled.

[t-b]

We now have SF_META_MOD_MARKER for exactly that.