Add icon4pygen namespace package

[samkellerhals]

Rename pyutils namespace package to icon4pygen since this makes more sense. Move the bindings subpackage inside icon4pygen since it is part of icon4pygen

[havogt]

I think it makes sense. Maybe we need to rework the repo layout a bit more.

[halungge]

So the idea would be that inside the pyutils src folder there could be several python packages, one of them being icon4pygen? That is, contrary to now there is no toplevel folder per python package but several of them can be grouped together.

I think this makes sense. We could in a similar way also group all the model code together (atm_dyn_iconam, advection, microphysics...)

[samkellerhals]

After speaking to @halungge I realised this is indeed more of a structural change than only renaming. However this could result in a more cohesive layout of the repository overall. The suggestion would be to have a more hierarchical setup of folders at the top-level containing the different importable packages. For example:

model/
    atm_dyn_iconam
    icon_driver
    ...

tools/
    icon4pygen
    liskov
    f2ser
    ...