Mixed type data returns from property calculations

[apontzen]

#211 provides a direct patch for an issue where "Nones" were being ignored when used in inclusion criteria.

However, the fix itself masks the deeper issue. If you gather a property which is some places stored as an integer and other places as a float, some of the data returned will be missing. (That is the origin of the Nones, at least in @TobiBu's case)

That would be extremely complicated to fix without degrading performance, so I am posting it here as a separate issue, hopefully to be addressed one day.