changed dynamic imports using exec to static imports

[clinssen]

This addresses #564

[terhorstd]

this one is related to #1058 and should be checked for compatibility

[babsey]

It looks fine to me and I do not know the reason why to import submodules dynamically (@jougs 8d8b19c).

However I approve it.

[jougs]

Is Topology still working with this fix in place? The reason for the dynamic import was to support extension modules that install an optional Python component. I don't see how they'd be working with a purely static import.

[clinssen]

Abandoning this PR due to on-point issues raised by @jougs!

We could potentially switch to a static import of the existing api files and a dynamic import in a special directory containing only user modules, but this is probably better handled later/as part of a different PR, because of active refactoring in PyNEST.

[babsey]

static import of the existing api files and a dynamic import in a special directory containing only user modules

I like the idea of having static and dynamic imports.

[apeyser]

It shouldn't be exec, but it shouldn't be static either, but should use importlib module (which probably doesn't fix PyCharm, but it's still cleaner than this).

Should be a different PR on this.