Add metadata to columns

It could be handy to add meta information to the columns of the DataFrames passed through the system. Think of information like:

• measurement frequency, important if different time series are merged
• variable type (precipitation, soil moisture)

I can think of several approaches to solve this:

• add the meta data to a columns MultiIndex
• store meta data in a separate dictionary and pass that around
• store meta data as part of the column name

Any suggestions @palmb and @luenensc