Small improvements to timeseriesaggregation

[ddceruti]

Fixed a Pandas FutureWarning, improved slightly two exceptions raised so that the complete traceback is preserved and deleted a unused import.

tsam\timeseriesaggregation.py:875: FutureWarning: A value is trying to be set on a copy of a DataFrame or Series through chained assignment using an 
inplace method.
The behavior will change in pandas 3.0. This inplace method will never work because the intermediate object on which we are setting values always behaves as a copy.

For example, when doing 'df[col].method(value, inplace=True)', try using 'df.method({col: value}, inplace=True)' or df[col] = df[col].method(value) instead, to perform the operation inplace on the original object.

[l-kotzur]

@ddceruti Thanks a lot for the update and the merge request.

@adbuerger mentioned further depreciation warnings with pandas 3.0. I will fix these together with this one.