refactor feature filter code

This an MR regarding issue #118 (closed)

Changes :

• Made a class out of filter.
• Renamed filter to Cleaner. Why ? filter is really too generic, the class name has to reflect what we are doing on the business object. And also imho handling missing value is not really filtering, it's more cleaning, but i could be wrong.
• Renamed pre_process_data to handle_missing_values. Again for the same reason. Now if someone see the code cleaner.handle_missing_values it gets more clear on what the function is doing without having to actually go into the code of it. For example if you read something like filter.pre_process_data, you have to go into the code to know what actually pre_process_data means.

Notes :

• Because Cleaner contains only one public method for now, i disabled the pylint error R0903. I think to add at least one more after this MR (dropping constants)