[Core] Optimize ai.wavelength setter

[EdgarGF93]

https://github.com/silx-kit/pyFAI/blob/main/src/pyFAI/geometry/core.py#L2955

That ai.reset() when setting the wavelength is not necessary, maybe a method soft_reset that keep the position arrays will do

[kif]

The different engines should nevertheless be reset, but some more arrays can be retrieved/saved