Relax assertions in preconditioner for ummatched weight and target sizes

[Fujikawas]

Main changes of this PR

The assertions in the preconditioner are relaxed to debug information to allow non-equal sizes of the weight and target to be scaled. This is the case for Wtil in #2010.
Tests are added for this change.

Motivation and additional information

Part of #2010.

Author's checklist

• I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• I added a changelog file with make changelog if there are user-observable changes since the last release.
• I added a test to cover the proposed changes in our test suite.
• For breaking changes: I documented the changes in the appropriate porting guide.
• I sticked to C++17 features.
• I sticked to CMake version 3.22.1.
• I squashed / am about to squash all commits that should be seen as one.

Reviewers' checklist

• Does the changelog entry make sense? Is it formatted correctly?
• Do you understand the code changes?