Mat: non-square block support for MatSetValuesBlocked

The old version always used row block size, which is incorrect for
non-square blocks.

Reported-by: Lawrence Mitchell <lawrence.mitchell@imperial.ac.uk>
Check column block size in MatSetBlockSizes

Previously, only the row block size was checked for consistency across
processes.
manual: fix typo mixing up mass and stiffness matrices

Reported-by: Gautam Bisht <gbisht@lbl.gov>
Merge branch 'jed/mat-setvalues-col-bs' into maint

* jed/mat-setvalues-col-bs:
  Check column block size in MatSetBlockSizes
  Mat: non-square block support for MatSetValuesBlocked