Fix #963: PETSc 3.20.0 broke the world

[guyer]

Fixes #963 (Actually, PETSC 3.19.0 broke the world.)

This PR:

• Assembles before multTranspose to prevent newly added exception

• Renames bandwidth to nonZerosPerRow and removes sizeHint

  The two were confusingly redundant:

  • PySparse takes sizeHint, the number of non-zeros in the matrix.
  • PyTrilinos takes NumEntriesPerRow.
  • petsc4py didn't used to be clear what it took, but is now documented as number of non-zeros per row (of the local portion of the matrix, but we'll ignore that part).
  • scipy doesn't preallocate.
  • Linear algebra defines bandwidth as "number $k$ such that $a_{i,j}=0$ if $|i-j| > k$", which is roughly half the number of non-zeros per row (and only applies to a band-diagonal matrix). Better to be explicit about what we really mean.

  Now all take same parameter and PySparse adjusts as needed.

  sizeHint was introduced in @a15d696 (in 2006!) to "allow smaller preallocations", but it was never used that way. Now, nonZerosPerRow can take an array_like to specify row-by-row preallocations, which are directly supported by PyTrilinos and petsc4py, and can be simulated for PySparse.

  Added exactNonZeros, which may have performance benefits for PyTrilinos and petsc4py. Currently unused.

• Uses Term's knowledge of own stencil to preallocate more effectively. Still doesn't do a good job with vector equations, but that's a deeper change (the resolution of which might help #920).

[guyer]

The one failure is due to an unrelated intermittent race condition

[guyer]

Now, the one failure is due to cosmic rays ☢️ 😠