SmallMatrix operator*= for MM

[ax3l]

Summary

Add a short-hand to multiply SmallMatrix with another value and assign the result.

Additional background

Often useful in ImpactX, e.g., latest is BLAST-ImpactX/impactx#1256

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate

[WeiqunZhang]

This only makes sense for square matrix, because the number of rows and cols are part of the type. So we should make that clear with enable_if when defining the function. We might also want to enforce that they have the same order and starting index.

[ax3l]

Ah you are totally right. Will improve accordingly.

[ax3l]

Don't need it after all.