fix inconsistent legends and ylabels

[orichters]

• Using the mip plot functions with piamInterfaces::plotIntercomparison lead to some strange ylabels and plot descriptions in mipArea / mipBarYearData / showAreaAndBarPlots: Confusing ylab in mipArea / mipBarYearData / showAreaAndBarPlots #60
• The reason was that shorten_legend was called with different settings, and that sometimes the back part was dropped completely
• I added a third option to shorten_legend where the ylab is provided and the plot variables are shortened by just cutting the ylab part out
• showAreaAndBarPlots() now derives ylab and passes it to mipArea and mipBarYearData
• Using the new function identifierModelScen(), harmonize that model names are shown in the legend only if different models exist. If all models or scenarios are the same, paste it after Model output such that it still present, but does not repeat endlessly in the legend (in particular with long scenario names).
• Harmonize parentheses around units to (unit)
• shorten_legend and identifierModelScen get extensive tests
• see below the things that were changed marked in red on the left, corrected on the right

[dklein-pik]

Very useful improvement of shorten_legend. Thanks!