The issue is now merged.

I left some comments on Bitbucket.

Simplifying a bit: if a template function is mentioned in the header file, we
must also define it there. So the main way to get rid of code is to avoid
templated public methods, and to keep those that we must have as simple as
possible. (The main thing that pimpl can do is to essentially hide away prive
methods.)

In any case, every step towards simpler *.h files is good, even if it is not a
giant step. :-)

On a side note: I did not touch the files option_parser.h and token_parser.h,
since Cedric and Patrick are changing those in issue826.

I had a first go at this an created a pull request:
https://bitbucket.org/salome_eriksson/downward-issues/pull-requests/1/issue827/diff

Sadly as far as I could see there is not too much that can be pulled in cc
files, since most functions/classes are templated. I did however not yet look
into the pimpl idiom too deep - could this be used to encapsulate the
implementation of templated stuff? I think somehow separating declaration and
definition of templated classes and functions would greatly increase readability.

In order to make the option parser code more readable, we want to split as much
code as possible off from .h files to .cc files. We possibly also want to apply
the pimpl idiom for this.

This is a subissue of issue588 (option parser refactoring).