Add the option to set apps in the machine file.

With this MR its know possible to set the used apps in the corresponding machine file. (As before: the machine file has to be named according to the hostname and this file is loaded automatically during the cmake process.) This can then be done appending the path to the application to the list USER_APPS. e.g.

list(APPEND USER_APPS "apps/gpu/LBM/WTG_RUB")

With that it is not necessary anymore to comment and uncomment the used apps in gpu.cmake. Here we only want to have a well defined set of sample applications like DrivenCavity.

In case of the usage of docker the hostname can be different:

• if the container is started manually with docker run the hostname can be passed as a parameter with --hostname <hostname>.
• when docker is used together with vscode the hostname can be set in .devcontainer.json (see in diff):

    "runArgs": ["--gpus","all",                     // remove this line in case you have no gpus available
                "--hostname=${localEnv:HOSTNAME}"], // HOSTNAME needs to be known by the vscode environment. It is probably necessary to add "export HOSTNAME=<hostname>" to the config file of your bash.

In this case it could be necessary to set the hostname in the config file of the bash as well. (e.g. .bashrc)

export HOSTNAME=<hostname>

• add wiki documentation here: https://git.rz.tu-bs.de/irmb/virtualfluids/-/wikis/Getting-Started-with-the-Development-of-VirtualFluids

Closes #30 (closed)