Create and implement a NERDataHandler class that establishes a common method to process labeled NER data in Spark NLP

[luca-martial]

The new NERDataHandler class will include:

• Write and read methods
• Storing docs indexes
• Easily filtering
• Converting inputs to match library requirements

It will go under nlptest/data_handlers/

The first version of this class will support robustness testing/fixing in Spark NLP, but adaptability to other tasks and libraries should be kept in mind when developing it.