getting through the masking:
- added some checks to process.pyx to get with zero mask
- created two-options dynamic masking in preprocess
- trying to get signal2noise ratio as a marker, together with the velocities equal to zero
fixed the masking: velocity is now zero and signal2noise is infinite means - MASK
- all the rest can be filled in by OpenPIV.
- yet need to think how to deal with the post-processing.
removed warning to error conversion