Remove using namespace from namespace level

[Flamefire]

There are several instances of using namespace X at namespace level which pulls in probably unexpected classes. Such things should be restricted to individual classes and/or at class level so the scope of this is limited. Although one could argue, that a using namespace PMacc in the picongpu namespace makes sense as both are very related, I would refrain from doing so. It recently led to a strange issue encountered by @Heikman where PMacc::FrameType (an enum) is unexpectedly used for FrameType (actual particle frame)

However this (https://github.com/ComputationalRadiationPhysics/picongpu/blob/4482d0bdc599e86bca2770c879b25030bf087aec/src/picongpu/include/plugins/hdf5/restart/RestartFieldLoader.hpp#L174-L175) is actually a sin! :smiling_imp: It pulls in PMacc into picongpu::hdf5 where it certainly does not belong.

I therefore suggest removing all using namespace X at namespace level, replacing class usages by fully qualified ones. If required, (local) namespace aliases or local namespace imports shall be used. This should also go into the coding conventions.

[PrometheusPi]

I totally agree - namespace renaming should avoid these problems and still allow short names for namespaces.