Preserve capitalisation of the model data in the output

[olejandro]

#259 introduced capitalisation of all input data which made the names of processes, commodities, user constraints, their description and units (incl. currency) look different in the output.

We could "undo" the changes after processing by using the names as they are before capitalising to create a dictionary which would be used to undo the change in the processed data. In case of duplicates, we should keep last.