Deal with external type bound procedures without local overwrite

[haraldkl]

An attempt to address #676.

[haraldkl]

This fixes #676, but not very elegantly. Somehow the externalization stuff bleeds into the templates/macros.html, which does not seem very desirable. The logic for external in the progress of the correlate methods may also be needed elsewhere, and maybe it would be better to overwrite the correlate functions for the External* classes.

[ZedThree]

Thanks @haraldkl! Not had chance to go through this properly yet, but it looks good so far!

[ZedThree]

This looks great, thanks @haraldkl! Sorry it's taken me so long to look at it. Does this change the format of the JSON file at all (that is, is it backwards compatible)?

[haraldkl]

Does this change the format of the JSON file at all (that is, is it backwards compatible)?

It only adds the deferred and attribs attributes to consider, but it should still work the same as before for existing JSON files (except that we do not recurse down the graph any further).

[ZedThree]

Thanks again @haraldkl, it's really appreciated!