Detect spectrum wavelength input units

[mchalela]

Provide a way to allow the wavelength to be in different units, like micrometers, angstrom, nanometers, etc. Maybe automatically detect the units in "read_spectrum" according to the order of magnitude of the spectral axis.

[Gaiana]

Also must include some estimation of the order of magnitude of the wavelength elements and compare it to an expected order of magnitude. Include a Raise error if the orders of magnitude do not match.

Also, include a warning when the dispersion is not linear. For this there must be some calculation of the differences between pixels.