hide/show the legend entries line by line

[TatianaBurek]

Describe the Enhancement

"We were making some plots in METviewer that use a lot of lines for ensemble members .

It ends up making a very large legend which then distorts the plot. It would be ideal to only include a representative member of each set of lines in the legend (i.e. 3 legend entries instead of ~30, so maybe the option to hide/show the legend entries line by line in the "Series Formatting" pane?). As a simpler solution, it would also suffice in this particular case to simply turn the legend off. "
Corresponding issue in METplotpy:
dtcenter/METplotpy#355

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[TatianaBurek]

The backwards compatibility of config files was discussed with users during RRFS Agile Framework Meeting (Nov.1, 2023).
Based on users recommendations the application should throw an error message if the required parameter is missing in the user config files. The application should not use default values to define a missing parameter.