NEW FEATURE: Let the user choose the strat plot identifier

[Chilipp]

Summary

The user should be able to select, whether the data shown when clicking Plot results is based on percentages or not.

Reason

Line diagrams and Area diagrams may both show percentage data (or not).

Detailed explanation

At the moment, area diagrams are always displayed as percentages data (see DataReader.strat_plot_identifier and line diagrams are not (see LineDataReader.strat_plot_identifier). There is however no reason, why this should in general be true.

Workaround

As long as this selection is not implemented in the GUI, this can always be changed from the console. E.g.

stradi.data_reader.strat_plot_identifier = 'percentages'

if the diagram shows percentage data (e.g. for Pollen diagrams), and

stradi.data_reader.strat_plot_identifier = 'default'

if not (e.g. for reconstructed temperatures, etc.)