Remove erroneous virtual tags in CsrMatrix

[alexander-novo]

Description

Closes #327

Proposed changes

N/A

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.
• I have updated CHANGELOG.md to reflect the changes in this PR. If this is a minor PR that is part of a larger fix already included in the file, state so.

(This could be considered part of adding the CsrMatrix class. Let me know if it needs a separate entry.)

Further comments