Allow imaginary frequencies when computing the spectral function along a path

[mesonepigreco]

Right now, if the code finds imaginary frequencies of the SSCHA matrix during the interpolation to compute the static response function, it crashes. However, sometimes it could be good to have the code ignore those frequencies just to print a meaningful output.
This is relevant when imaginary frequencies occur close to gamma (acoustic modes) due to interpolation issues.
See this discussion:
SSCHAcode/python-sscha#74