Conf-Import fails without error if single cell contains string

[philippkraft]

We need a clear warning if a column that is defined for import is of dtype 'object'. If that is the case, the none float values in the column should be found and listed, with the suggestion to move them to the No-Data list.

[philippkraft]

In pandas_import.load_dataframe we need a check of each Value-Column like:

for col in df.columns:
    try:
        df[col] = df[col].astype('float')
    except (ValueError, TypeError) as e:
        warnings.append(f'Column {col} cannot be imported: {e}')

[philippkraft]

Return value of load_dataframe should be changed to df, warnings and return the dataframe together with a list of warnings. Need to check all places where load_dataframe is referenced.

`load_dataframe' is called from

• webpage.filemanager.dbimport
• submit(...)
• test_di_pandas_import
• interactive_import

[philippkraft]

Check if really all columns in load_dataframe must be float