Makes `iterator` a method

I removed the `iterator` from the Synfit class and tranformed it into a
method.

I also turned the variable `iter_values` unbound to the class (removed
the self)
Deep copies dictionaries

By using the built-in copy function of dictionaries, a dictionary inside
a dictionary is not copied and just referenced. To solve this, it is
needed to deep copy it.
Starting sketch a library builder

This function will create a library of not convolved spectra to be used
when iterating.

Right now, it only works if the parameters to be fitted are only 'vrot'
and/or 'vmac_rt'.
Replace key to be looped

Since the array with the values to be iterated were constructed using
the fit_keys, I can use the keys on the structured array directly.
Add decorator to function

The merged_abundances is a static method function.
Segregate variables and add guideline

I tagged the parameters to be fitted as rotational and non rotational.
This will help guide which kind of spectra library should be build. At
this point, it only builds a library if there is is only rotational
parameters.
Implement library build for rot+ non rot

It the parameters to be fixed are a mixture of rotational and non
rotational parameters, it will build a spectral library without rotation
and then convolve on these spectra.
Implement fits of non-rotational parameters

If there is only non rotational parameters, the library is not built.
Merge branch 'feature-fitting-convolution' into feature-fitting

The purpose of these branch was to speed up Synfit by allowing it to create
 a spectra library with vsini=vmac_rt=0. Then, it would just convolve these
 spectra, what speeds up.