Errors found in the `input_output` notebook

[despadam]

• [ ] Add a line to execute solution_print_salutation(), so they can see the output, since this one cannot be tested normally

[yakutovicha]

The exercise solution_find_all_files has issues:

• We provide the following solution list(f.parent.iterdir()) but it should not have parent in it.
• The path argument in our solution should be current_path instead of f for consistency.

[despadam]

In writing to a file section:

• we have 3 exercises but they are enumerated wrongly (instead of 1,2,3 we have 1,2,1)
• solution_read_file should be removed from this section. It is a duplicate

[despadam]

In some reference solutions we already use context managers before they are later introduced. Let's update those solutions to use the syntax where we manually close a file at the end. Then we can add a new exercise in the context managers section to introduce the new syntax

[despadam]

In reference_read_write_file we are using line.strip() and len(line) for a string. However we haven't introduced any of these two in the section for strings in basic_datatypes