Unsolved tutorials - remove answers and add comment prompts to tutorials 12-15

Tutorials 12 to 15 are new and don't have an "unsolved" state yet - it would be an improvement to remove the answers from these tutorials in the 'unsolved' folder and add hints for how to solve the tutorials instead. See Tutorials 0 to 11 for examples of how to do this - e.g.

This is the "solved" version of a cell in tutorial 10:

```python
r=l.calculate_enrichment(widget="publication_enrichment",maxp=0.01)
```

and this is the 'unsolved' version:
```python
# The syntax to get enrichment stats for a list is as follows:
# my_list.calculate_enrichment(widget="widget_name",maxp=some_max_p_value)
#
# Try getting "publication_enrichment" with a maximum p value of 0.01.
# store it in a variable called enrichment_results
```

Inspect the unsolved versions of tutorials 12-15 and compare them with tutorials 0-11 for the expected hint style. Replace answer cells with comment prompts that guide each exercise without revealing the solution. Done means all four tutorials have matching unsolved states and useful prompts.

Scritto dal modello di indicizzazione a partire dal testo della issue.