Fix Fourier ratio default treshold

EELS.fourier_ratio_deconvlution bug: Docstring say default treshold of
None should make method use first minimum. The feature was not
implemented, and caused an exception in
estimate_elastic_scattering_intensity. Fixed by calling
elastic_scattering_treshold if treshold is None.
Merge pull request #227 from vidartf/VidarsBranch

Fix Fourier ratio default treshold