add flags and cell values to integrate_ppi() raster output

[adokter]

Depends on completion of adokter/vol2bird#129

• flags will be integrated over scans using a bitwise XOR operation
• cell values will be merged over scans.

[adokter]

@bart1 @BerendWijers is this one very high on your list still at the moment? We talked about this when I was at UvA and promised to work on it before going to the radar aeroecology conference in China, but urgent paper revisions took all my time. Trying to assess where the priorities are for you.

[BerendWijers]

@adokter I've discussed it with Willem - and it indeed is very much on our wish list. Considering we've completed quite some work with the range bias corrections - we would love to continue with clutter visualizations. Furthermore, we understand that you are busy and that you are doing us a great favor. If there is anything we can assist with in order to reduce the load for you please let me know.