Add intersphinx mapping

[danielhuppmann]

This PR adds the intersphinx mapping to pyam and restructures the docs (rst pages and code docstrings) to use intersphinx markup.

[phackstock]

Very nice addition of referencing both within the package and to pyam as well.
Right now, the link for ":class:RegionProcessor" is not working because in the api docs I included it as "automodule:: nomenclature.processor.region" and not "autoclass:: RegionProcessor". Should we fix it here or should I address it in #76?

[LauWien]

Looks great :)
Just a few suggestions inline.

[danielhuppmann]

Response to @phackstock:

Right now, the link for ":class:RegionProcessor" is not working because in the api docs I included it as "automodule:: nomenclature.processor.region" and not "autoclass:: RegionProcessor". Should we fix it here or should I address it in #76?

I think a priority is to make intersphinx operate smoothly also for other users pointing to our docs, so we should make sure that :class:nomenclature.RegionProcessor just works. So we should restructure the API documentation (ideally as a separate later PR - it's not a big problem if the link is not created right now).

[phackstock]

@danielhuppmann ok, sounds good. Then good to merge from my side. I'll rebase #76 once we've merged.