Implementation of BLR constructed of multiple shells

[jsitarek]

One thing that we discussed yesterday during the call was to implement a BLR composed of multiple shells. I thought how to do it in the code in the most clean way, and I would suggest such a solution

1. create a new class (e.g. BLRShells) initialized only with disk luminosity
2. the new class will have store a list of SphericalShellBLR objects and we should add methods to add/remove a single line and add the whole list of lines (from lines_dictionary)
3. in the absorption.py we add opacity_shells_blr which simply make a loop over corresponding opacity_shell_blr (this function takes self as the first parameters, but I hope you can still run it over a specific target in an easy way)
4. similar things we can do also for IC

@cosimoNigro what do you think?
@pawel21 could do the actual implementation