Improve UserProvidedMarketData input validation

[enzbus]

Connected to #173

Main reason why UserProvidedMarketData is not shown too much in the examples is that it's harder to guarantee that things will work properly with arbitrary user inputs. DownloadedMarketData enforces a plethora of data consistency properties. We should be able, however, to give more feedback on at least simple things that are definitely errors, like index/columns duplicate, non-monotonous time indexes, misalignments of returns and volumes, .... Something is there already but not enough. #173 is a good example of a valid usecase by expert user who ran into issues. An example script with detail comments should be provided.