Initialize empty parameters inside __init__

It was initializing some parameters outside __init__. It seems that this
is not a good programming practice.
Adds function to plot best fit

The Synfit object has now a module to plot the best fit.
Check if the input parameters are correct

It checks if the input parameters are correct. If not, throws an error
with an explanation.
Removes check for radial velocity

In the __init__, it already sets the value of radial velocity, so there
is no need to check it.
Improves plot

The plot of each iteration now shows the value of the calculated
chisquare. I had to do an workaround so the synthetic and the observed
spectra would match in scale and radial velocity.