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
```