Setup continuous integration for NEST GPU repository

[jhnnsnk]

Analog to the NEST Simulator repository, we would like to setup a continuous integration pipeline with static checks, building of NEST GPU, and unit testing, see Documentation.

NEST GPU already has a number of tests available, and we need a backend with GPUs for running these tests.

We would appreciate suggestions and support from the experienced NEST developers for setting up such a pipeline, @heplesser @jougs @clinssen @terhorstd.

[jhnnsnk]

@clinssen mentioned the new possibility to have GitHub runners on JUSUF with access to GPUs.