add missing xyz output format for helium and fixes of corner cases

[hforbert]

No description provided.

[oschuett]

You can use:

 CALL cp_warn(__LOCATION__,"your lengthy message, "//&
              "which can even span multiple lines")

See also: https://www.cp2k.org/dev:error_handling

[hforbert]

Thanks, fixed it the way elsewhere in that file.