Consistency checks for basic FMU properties

[ghorwin]

Implement consistency checks:

1. FMU name must not be empty
2. FMU name must not hold characters that are forbidden as file names
3. target directory must be a valid path
4. if target path does not exist, yet, user must be asked to give permission to create the basic directory
5. target subdirectory for generated FMU must not exist as file
6. if target subdirectory for generated FMU exists (as directory), user must be asked to give permission to remove the old directory structure before creating the new one