Add method to set meta indicators from data

[danielhuppmann]

It would be useful to have a generic df.set_meta_from_data() function, e.g.,

df.set_meta_from_data(name, method, column='value', **kwargs)

where

• kwargs can be additional filters (for example apply only where variable='Temperature' or region='World')
• method can be min, max, mean, median, and maybe even other pyam functions like pyam.timeseries.cross_threshold() or cumulative()
• column would be what to set as meta - the actual value (default) or the year/region/... column where the value is reached (e.g., year of peak temperature)
• name is the column name in meta (similar to the use in df.set_meta())

So the use case suggested by @znicholls for setting the peak temperature (see #237) could be implemented as

df.set_meta_from_data(variable='Temperature', name='Peak temperature', method='max')

[danielhuppmann]

closed via #241