Add PhononDos.mae() and PhononBandStructure.has_imaginary_gamma_freq() methods

[janosh]

@JaGeo Would be great to get your review on this, esp. the new PhononDos.mae() and PhononBandStructure.has_imaginary_gamma_freq() methods.

[JaGeo]

Have you checked the tolerance in detail? I think it might be very tight. Similar for the overall tolerance

[janosh]

I just copied the tol=1e-5 from the existing has_imaginary_freq method but use tol=1e-3 in my own analysis. Should we increase to tol=1e-3 by default?

[JaGeo]

I just copied the tol=1e-5 from the existing has_imaginary_freq method but use tol=1e-3 in my own analysis. Should we increase to tol=1e-3 by default?

Yes, this would be great!

I haven't spotted anything else when reading the pull-request. However, I haven't tested it in detail.