Refactor/sort

[cs168898]

Sort method refactorization

• Refactored sortValues method to handle multiple columns, used for tie breaking columns.
• Refactored sortValues method to handle Nil value scenarios.

Example Syntax:

colNames := []string{"score", "score2"}
df.SortValues(colNames, ascending)

where colNames is the slice of columns to sort by.

Important things to note**

• Nil value rows will be pushed to the last row of the dataframe.
• Each time the values are identical, it will move onto the next column to compare provided in the slice.

#26