replace smoothn.py with scipy.interpolate

[alexlib]

smoothn.py is quite old and not Pythonic
The file is a copy from 2010 (copyrighted by the original author and translated in Python 2.7 from Matlab) and it's quite anachronistic in Python 3.10 or 3.11

**scipy based robust smoothing **
PIV requires a scalar filed smoothing with noise and gaps. Find the scipy`` or other tools that do the job similar to smoothn.pyfor instance there is asmoothfit` package that apparently does a good job and we need to adjust it to use with OpenPIV