Introduce attribute filtering into DEMRasterSource

[echeipesh]

Connects: #71

In the above issue I said:

Normally it would be possible to filter point cloud to remove canopy returns. However, the result above was generated from the intermediate levels of the EPT pyramid.

This is potentially a lie, we have not actually tried doing attribute filtering at those zoom levels. It could be that due to sample size there are already enough samples at given zoom level that are ground returns. Practically speaking we could already be able to return much better product.

This issue is to add attribute filtering and evaluate the results on 3DEP datasets before the larger work of #71