Simplify optdone_as_list and add future command line option that implies it

I looked this code over one more time and I think it can be simplified, in terms of any future changes and reverts. We want to be able to get rid of this quickly if we want, so subclassing ccData is the way to go -- the modifications are then very clear, and the only code that deals with optdone_as_list are in the constructor of Logfile, where a different datatype obejct is chosen.

Also, added a 'future' option to ccget that implies optdone_as_list, so we cann attach more functionality to that if we want to test things when breaking the API. And it works:

```
kml@anula:/usr/local/src/cclib$ ccget optdone data/Gaussian/basicGaussian09/dvb_gopt.out 
Attempting to parse data/Gaussian/basicGaussian09/dvb_gopt.out
optdone:
True
kml@anula:/usr/local/src/cclib$ ccget --future optdone data/Gaussian/basicGaussian09/dvb_gopt.out 
Attempting to parse data/Gaussian/basicGaussian09/dvb_gopt.out
optdone:
[4]
```