Merged and pushed

Feel free to merge, this doesn't need another review.

Comments incorporated

I had a look and think the code looks good (even better after the minor comments 
are addressed :-)).

I had a quick look, but I'm tired, so it'd be good if someone else could have a
look, too. :-) Looks good to merge for me.

Here is the pull request:
https://bitbucket.org/cgeissmann/downward/pul
l-requests/8/only-use-one-predefinitions/diff

On the task board it says "waiting for review". Could you please post a link to 
the pull request?

Currently, the class that stores predefined objects in the option parser (like
predefined heuristics) is templated. We want to get rid of the templatization by
having one non-templated class containing a map which maps type_index to a
storage object which in turn stores all objects of this type. An example of how
this can work is already implemented in PluginTypeRegistry.