sigma2FWHM function doesn't handle lists properly

[hoogerheide]

If wavelength, wavelength_resolution, angle, or angular_resolution are defined as lists in the header of a 4-column data file, they are passed to resolution/Sigma2FWHM as lists. Multiplying by 2.355 in Sigma2FWHM simply doubles the length of the list instead of multiplying the values elementwise. Suggested change would be to replace "s" with "np.asarray(s)" in lines 55 and 59 of resolution.py