Support of shell user element (US3)

[FEAnalyst]

@synt

maybe an option to overwrites from ‘S3’ to ‘US3’ is possible, so a user not required to exit.

Currently, it’s easier to export the input file and modify it outside of PrePoMax for this purpose. Otherwise, keyword editor allowing modificafions of existing keywords (as discussed here: Keyword Deactivation) would be needed.