Replace hand-written modelsmodule by a code generation framework

[jougs]

This replaces the hand-written modelsmodule.cpp by a mechanism to generate it dynamically from CMake arguments.

I think the documentation I wrote best explains the rationale for this change:

By default, NEST will compile and register all neuron and synapse models that are shipped in the source distribution. This is very convenient for an explorative development of simulation scripts, but leads to quite long compilation times and is often not necessary.
Now, there are two ways to restrict the set of built-in models to tailor NEST to your needs:

• -Dwith-modelset=<modelset> allows you to pecify the modelset to include. Sample configurations are in the modelsets directory in the top-level of the source tree. This flag is mutually exclusive with -Dwith-models. [default=full].
• -Dwith.models=<model_list> allows you to specify the models to include as a semicolon-separated list. This option is mutually exclusive with -Dwith-modelset [default=OFF].

This is the better solution I have promised in #2663.

[jougs]

@JanVogelsang: Voilà! Only nine weeks after I said "this is really easy!" 😇 😅

[jougs]

@jessica-mitchell: It seems that all links to the Potjans model of the form :doc:`microcircuit model <../auto_examples/Potjans_2014/index>` are broken. Is this related to this change?

[jessica-mitchell]

@jougs, hm they seem to work for me both in this PR and master: I checked the link from models-main pageand the link in the examples index page and both correctly displayed the Microcircuit model docs.

[jougs]

@jougs, hm they seem to work for me both in this PR and master: I checked the link from models-main page and the link in the examples index page and both correctly displayed the Microcircuit model docs.

Are you using a build directory separate from the source directory or are you running cmake right in the Git clone?

[jessica-mitchell]

Are you using a build directory separate from the source directory or are you running cmake right in the Git clone?

I was checking the build on Read the docs, but the local build (current master) with a separate build directory also works for me.

[pnbabu]

Looks good to me!