Fix errors during configuration update when reading obsolete options.

[phw]

This fix makes old confiuration options, for which there is no Option instance in the code anymore, accessible.

Fixes PICARD-642.

@mwiencek: Since you reworked the parts of the code that finally led to this, you maybe have some better idea how to implement this.

[phw]

I have reproduced the issue and tested the fix on both Windows and Linux by using pre 1.0 versions of Picard to generate the configuration and then running the latest code.

For reference the commits that changed the option loading (and thus created this problem) were:
6ff1f53 (this just started the refactoring)
c50cdf7 (this finally prevents loading unregistered options)

[mwiencek]

Looks good to me. My only concern would be if type.convert throws an exception for some reason. I can't think of any reason why it would, but the states peoples' configurations end up in always surprise me.

[mwiencek]

I guess one example would be if the options don't exist for some reason and value is None or an empty string. Maybe it'd be safer to factor out lines 47-55 (which checks self.__config.contains) and use that as the basis for the new method.

[phw]

Thanks for the feedback, I applied some refactoring. This really makes this code much better, the value method and the [] operator now share most code, the method is just more generic and doesn't use the option registry.

[mwiencek]

Looks great 👍

[zas]

Can i merge it to master ?

[phw]

It's ready for merging.

Can i merge it to master ?

Either that or merge it to the 1.3.1 branch and later merge that branch back to master.