merge existing tools for data upload

Actually we have two ways of uploading data: import from csv UI and python script for massive import. I propose to merge theme, forcing the first one to use the python script. Doing so we will gain also the possibility to overwrite existing data when time steps are coincident, since this is actually possible with the python but not using the UI. Possible workflow:
- keep the existing upload csv page
- on click of upload button, all data eventually present in the destination folder of the upload are removed so that after the upload only that single file is present
- on click of "verify and import" button the python script is called, with the destination folder as argument and it imports the file