Unifying output of OpenPIV

[alexlib]

Is your feature request related to a problem? Please describe.
Today OpenPIV output is in the .vec ASCII 5 column files: x,y,u,v,quality. However, depending on the user inputs these outputs get few confusing units options, either pixels and pixels/dt or pixels/second (if dt = real value of time in seconds) or meters and meters/second, if the user provides both the real scaling argument and real delta t, and I think all the weird combinations are possible, if one of these parameters is a dummy value.
The situation will get even worse once we implement @TKaeufer work on the calibration using target images and workign with oblique PIV views and stereoscopic PIVs. Then the image to world calibration would create always the output in meters and the only missing piece of information is the delta t. So what I see is that depending on the settings and user inputs, one could get to different results and probably get confused.

Describe the solution you'd like
Either we take care of creating very clear set of instructions and the output file would contain the units (e.g. TSI Insight does that and we somehow copied their behavior with the header), or we try to get rid of some assumptions and force one type of the units.

Obviously this is not a solution, but just an invitation for discussion