Rename .aggregation() to .aggregate()

[equinor-ruaj]

.aggregate() creates a new aggregation always.
.aggregation() first tries to get an existing aggregation, if not it creates it.

.aggregate() should do what .aggregation() does today

Deprecate .aggregation()

Possibly also add a parameter force=true|false to force creating a new aggregation

[liidenee]

Wait a bit with this task, need more information to decide the approach.