Feature/drop duplicates

[cs168898]

Drop Duplicates method

• Added Drop Duplicates method to allow users to drop any duplicate rows in the dataframe
• Added DropDuplicatesOption struct to allow users to further fine tune the drop duplicates method which allows the options of:
  Subset -> Custom column names to check for duplicates
  Keep -> To keep which row when there is occurrence of duplication.
  Inplace -> To allow users to select whether to modify the existing dataframe or create a new dataframe with the modified rows.
• getRowKey method is implemented to get the unique key of the row where it is made up of the column names and values in string format to keep in a map to recognise duplicates.
• getSubslice method is implemented to get specific rows of the columns based on the row indexes to keep that is decided based on the logic before this method was called.

Syntax:

// To define the options:
options := goframe.DropDuplicates{
Subset : []string{} // Etner the column names in string format to check for duplicates
Keep : string // Enter "first" , "last" or "none"
Inplace : boolean // Enter true or false, true will modify the existing dataframe while false creates a new dataframe with modified rows
}
// To call the method:
df.DropDuplicates(options ...DropDuplicatesOption) 

Important things to note**

• If no options is added to the arguments when calling the method, the default options are:
  Subset -> All columns
  Keep -> Keep only the first occurrence
  Inplace -> False, creates a new dataframe without the duplicate rows.

Things to improve

1. Ignore Index option should be implemented where users can select if they want to re-index the dataframe post method call.
   please add sort,describe,toDatabase,readFromDatabase,dropDuplicates #26