Decouple non-modular if-else statement from the AggregatorParser

[jonasvdd]

#154 added a new hierarchy-based aggregator class structure, which requires different down sample implementations

Currently, this is resolved by an if-else statement in the PlotlyAggregatorParser its aggregate method, which is not that clean / modular.

• Consider splitting the downsample and datapointselector into two different fields
• that way you can avoid all of this switching and have well-defined types. TODO:
• [ ] add link to code location where this switching occurs.