Add Min/Max/Avg graph degree aggregation (static and temporal)

[ChrizZz110]

STATIC
A graph aggregation function that calculates the min/max/avg degree of a logical graph. The aggregates are added as properties to the graph head. Each aggregate results in one scalar value.

TEMPORAL
A graph to value operator that calculates the min/max/avg degree of all vertices of a temporal graph. The result is a dataset of Tuple3<Long,Long,Double (Avg) / Int (Min/Max)> where the tuple positions represent the following:

1. the interval start
2. the interval end
3. the aggregated value for that interval