0.16.0 - S, SS and Expressions

New Features

• Created S object (represents $ sign since it is not a valid variable name in python) to store all MongoDB operators and to create references to fields
• Created SS object (represents $$) to store aggregation variables and referenes to user variables
• Interfaced a chunk of new operators(add, divide, multiply, pow, substract, cond, if_null, switch, millisecond, date_from_string, date_to_string, type_)
• Integrated new operators in Expressions class

Refactoring

• Redefined Expressions completely. Simplified and clarified how they can be used when using this package.
• Removed index module that was at the root of the package (monggregate.index.py -> ø)
• Removed expressions subpackage (monggregate.expression -> ø)
• Moved expressions fields module to the root of the package (monggregate.expressions.fields.py -> monggregate.fields.py)
• Removed expressions aggregation_variables module (monggregate.expression.aggregation_variables.py -> ø)
• Moved the enums that were defined in index to a more relevant place. Ex OperatorEnum is now in monggregate.operators.py

Breaking Changes

• Operators now return python objects rather than expressions/statements.
  NOTE: The wording might change for clarification purposes.
  statement might be renamed expression and resolve might renamed express
  To do so, some arguments might need to be renamed in the operators
• Expressions subpackage has been exploded and some parts have been deleted

Documentation

• Updated readme to reflect changes in the packge. Readme now focuses on the recommended way to use the package and clarifies how to use MongoDB operators.