…egation $group stage.

Original Pull Request: #360
CLA: 171720160409030719 (Gustavo de Geus)

Just added overloads for stdDevSamp and stdDevPop taking AggregationExpression and updated the doc.
Also replaced String operation based MongoDB operation building by using operators directly.

Original Pull Request: #360

We new support $filter in aggregation pipeline.

Aggregation.newAggregation(Sales.class,
	Aggregation.project()
		.and(filter("items").as("item").by(GTE.of(field("item.price"), 100)))
		.as("items"))

Original pull request: #412.

Remove variable before returning value. Add generics for list creation.

Original pull request: #412.

…roup & project.

We now directly support comparison aggregation operators ($cmp, $eq, $gt, $gte, $lt, $lte and $ne) on both group and project stages.

Original pull request: #414.

Add JavaDoc for compareValue.

Original pull request: #414.

…line operators.

Original Pull Request: #410

Add missing transformations for ConstructorReference, OperatorNot, OpNE, OpEQ, OpGT, OpGE, OpLT, OpLE, OperatorPower, OpOr and OpAnd. This allows usage of logical operators &, || and ! as part of the expression, while ConstructorReference allows instantiating eg. arrays via an expression `new int[]{4,5,6}`. This can be useful eg. comparing arrays using $setEquals.

More complex aggregation operators like $filter can be created by defining the variable references as string inside the expression like filter(a, 'num', '$$num' > 10).
Commands like $let requires usage of InlineMap to pass in required arguments like eg. let({low:1, high:'$$low'}, gt('$$low', '$$high')).

Original Pull Request: #410