Create a Common function for exporting dataframes

[MauroPen]

The export function is often used: therefore, it makes sense to generalize its formulation and include it in the Common.py dependency.

Exporting is characterized by the following:

• A name for the file with the time of production --> A static string + a datetime.now() object
• One or more dataframes to export --> Meaning one or more sheets to be generated in the final file
• Ore or more names to be assigned to each sheet --> One string for each dataframe to export
• One or more boolean parameters to indicate how to treat indexing --> One boolean for each dataframe

For this reason, it might make sense to define a new class of objects called ExportDataFrame with the following attributes:

• Dataframe: the DataFrame object that has to be exported in a sheet of an Excel file
• Sheet Name: the string that will be set as the name of the corresponding sheet
• Indexing: a boolean parameter providing indications about how to treat indices

Therefore, the function is expected to need the following arguments:

• nameOfFile: the name that we want to give to the file
• timestamp: the time when the export process started
• dataframes: an array of ExportDataFrame objects

[MauroPen]

The export_dataframes() function (#4), together with its Class DataframeExport (#3) has been added to Common.py