Replace sys.exit with h.quit to fix issue with MPI_Finalize

## Context
With `sys.exit` the application was forced to exit and didn't call `MPI_Finalize` in the end which especially with `singularity` generated problems.
Now with `h.quit()` `MPI_Finalize` is called and it also returns the proper exit value to the shell.

## Scope
Neuron exit codes and MPI Finalize

## Testing
Should be tests by the CI.

## Review
* [x] PR description is complete
* [ ] Coding style (imports, function length, New functions, classes or files) are good
* [ ] Unit/Scientific test added 
* [x] Updated Readme, in-code, developer documentation

init.py

@@ -6,6 +6,7 @@
 """
 import sys
 from neurodamus import commands
+from neuron import h
 
 
 def main():
@@ -31,5 +32,5 @@ def main():
 
 
 if __name__ == "__main__":
-    # We truly require this so that, when launched with special, the process finishes
-    sys.exit(main())
+    # Returns exit code and calls MPI.Finalize
+    h.quit(main())