suggestion for normalizing continuum

[arashgmn]

I think it's better to have an expression like zeros = np.abs(continuum_flux <= 1e-4) would be more robust to round-off errors. Just as a suggestion.

https://github.com/marblestation/iSpec/blob/1d6482857705db12131ed6b5dec486dcccd5cac1/ispec/spectrum.py#L406