Possible dependence to frame of references

[mkraposhin]

In file fwhFormulation.C I found next lines:

https://github.com/unicfdlab/libAcoustics/blob/b9cd1d62ad8c5b445240a506a945231d12454b73/Sources/lib/analogies/FfowcsWilliamsHawkings/fwhFormulation.C#L73

https://github.com/unicfdlab/libAcoustics/blob/b9cd1d62ad8c5b445240a506a945231d12454b73/Sources/lib/analogies/FfowcsWilliamsHawkings/fwhFormulation.C#L76

On these lines components of vectors r_ are used explicitly, meaning that libAcoustics is sticked to specific frame of reference. I think, that should be avoided, because we don't know in advance what frame of reference will be used by the library users.