Add field for rotation speed in RRDEs and flow rates for flow cells.

[DunklesArchipel]

During RRDE measurements, the sample is rotated at various speeds, and in flow cell experiments, different flow rates can be employed. Both can have an impact on the measured current densities.

[linuxrider]

We could add a property convection which applies either to the working electrode with rotation or the electrolyte.

[DunklesArchipel]

See echemdb/svgdigitizer#260 where rotation: 30 min-1 was added to SVGs.