Another commit related to timezones... The Unix date always being in UTC, this has been fixed. The resampling was not using dates in a proper way, this has been fixed. The GSN plugin has also been fixed, it now rounds time to the closest minute (in order to be consistent with our advertised "1 minute time resolution"). A rounding method has been implemented for dates.

The timezone parameter in io.ini is now named TIME_ZONE instead of TZ for clarity. 

Undefined dates should now be fully handled (ie: also in arithmetic).

Finalization of the new filtering infrastructure: speed optimizations (IOManager, MeteoProcessor), clean up

New filter system rolled out with currently two filters up and running: min_max and mean_avg (without the soft option, only left or right centering). The new filter system is built upon processing blocks that take a vector<MeteoData> as input and return a processed vector<MeteoData>. The individual filters are stacked in a ProcessingStack where they are invoked sequentially.