incorrect rounding in scaled_rounding with scale of 1

[dhebbeker]

Most rounding functions in scaled_rounding don't work as intended if SCALING==1. I suggest to

• either forbid to use a scaling of 1:
  • note in the documentation
  • static_assert in the code
• or use template meta-programming to return value if SCALING==1

Can you please clarify which behaviour is intended? Then somebody could start a pull request...

_{Based on ETL version 20.40.0.}