Add Unit and Denominator to Panda DataFrames

[komape]

The conversion of a timeseries object to a Panda DataFrame should contain the unit and denominator. This is particularly useful since the introduction of unit conversion for multiple endpoints. The name of the curve (which is included as header in a DataFrame) contains the unit but the data has been converted to another unit. The main issue is the question how to integrate the unit in a DataFrame. As a header could be one option. Another option is to update the curve name of the DataFrame.

Another option could be to add the unit in brackets to the column name. For example DE Wind ... (GWh/h).