More functions to be parallelized

[crisluengo]

~~ProjectionScan (source), which is used by all projection statistics functions such as dip::Mean and dip::Percentile, needs to be parallelized by adding an OpenMP pragma to the loop. This is not very hard, but not trivial either.~~

dip::SelectionFilter (source) can also be easily parallelized, following dip::Framework::Full.

dip::ChordLength, dip::PairCorrelation, etc. can all be parallelized quite simply, I think.