Make main.py executable and create a symlink during installation

[tmadlener]

BEGINRELEASENOTES

• Mark main.py as executable and create a symlink from <prefix>/bin/k4GeneratorsConfig to <prefix>/python/main.py during installation. This makes it possible to setup an environment where k4GeneratorsConfig can be used as a simple executable
• Remove some PYTHONPATH modifications from within the interpreter and use absolute imports. This makes it possible to simply add <prefix>/pyhton to the PYTHONPATH and everything will work.
  • Now we use relative imports internally mostly and make sure to expose Generators.Generators via settings in __init__.py

ENDRELEASENOTES

One thing I discovered that simply calling <prefix>/bin/k4GeneratorsConfig without argument, leads to:

$ ./install/bin/k4GeneratorsConfig 
Generating and writing configuration files
Traceback (most recent call last):
  File "/home/tmadlener/work/key4hep/k4GeneratorsConfig/./install/bin/k4GeneratorsConfig", line 158, in <module>
    main()
  File "/home/tmadlener/work/key4hep/k4GeneratorsConfig/./install/bin/k4GeneratorsConfig", line 102, in main
    executeFiles(files, 0, rndmSeed, events)
  File "/home/tmadlener/work/key4hep/k4GeneratorsConfig/./install/bin/k4GeneratorsConfig", line 154, in executeFiles
    return rndmIncrement
           ^^^^^^^^^^^^^
UnboundLocalError: cannot access local variable 'rndmIncrement' where it is not associated with a value

Ideally this should simply spit out a usage message, rather than error. Things work with -h or --help. Maybe there is an easy way to fix this, by e.g. making some of the arguments required, in which case argparse would do the right thing.

• name of k4GeneratorsConfig symlink? See also dicsussion in Add k4GeneratorsConfig package and include it in key4hep-stack key4hep-spack#617
• Also update the setup.sh script?

[dirkzerwas]

Protection added against -f FILENAME absent on command line

[tmadlener]

Is not passing an input yaml file a "user error"? I.e. is the expectation that there should always be at least one yaml file? Because then we can just change nargs="*", to nargs="+".

[tmadlener]

Suggested change

	link_name=${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_BINDIR}/k4GeneratorsConfig
	link_name=${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_BINDIR}/k4generatorsConfig

I just realized that the alias in setup.sh is capitalized this way. I have no preference either way, it just has to be consistent in the end. @dirkzerwas any preferences from your side?

[dirkzerwas]

Your choice makes more sense, let's leave it as "k4GeneratorsConfig" , I will update the setups and tests to be consistent.