Add text label `rpm` to echemdb cv plots

[DunklesArchipel]

So far echemdb metadata schema does not consider rpm https://github.com/echemdb/metadata-schema/issues/52

Once this is implemented we should add the possibility to extract these from the rpm rate for RRDE experiments from the SVG.

[DunklesArchipel]

Added rotation: 30 min-1 and rotation: 80 min-1 in the SVG of howlett_2006_electrochemistry_1483 in https://github.com/echemdb/electrochemistry-data/pull/89. This can be changed laster and should be extracted once the metadata schema has been adapted for this key (see https://github.com/echemdb/metadata-schema/issues/52)