[Visualization for functions] Log history and modify the visualization for the `describe()` call for a Series

[leijie-wang]

Describe should work in a similar way as value_counts and unique, that is, whenever a describe is called for a child series, we should register this event to the parent dataframe. In this way, we won't lose track of the function call when the user writes codes like df["Cylinders"].describe() instead of df.describe().

Besides, for the function call df["Cylinders"].describe() , the resulted visualization is actually a histogram instead of a boxplot as expected.

[leijie-wang]

Solved in the describe-improvement branch (which is based on the unit-test-implement branch). Waiting to be examined and merged.

[leijie-wang]

See #12 for more details about this branch