Generic LinearOperator.toarray() and tosparse() defaults

[spossann]

Core changes:

• LinearOperator.toarray(out=None, is_sparse=False, format='csr') is no longer abstract. The default builds the global matrix one column at a time, as dot(e_j), so it works for any operator, including matrix-free ones. It supports StencilVectorSpace domains and (possibly nested) BlockVectorSpace domains made of them. It runs in serial and in parallel (every rank gets the full matrix) and returns either a dense array or a scipy.sparse matrix. This is ported from struphy's LinOpWithTransp.toarray_struphy(), with these improvements:
  • nested blocks are supported, with one code path for Stencil and Block domains;
  • the dense result is summed across ranks in place (Allreduce(IN_PLACE)), so no second full-size array is allocated;
  • one ghost-region update per column instead of two;
  • the sparse result is collected with one allgather per array, and the format is converted with coo_matrix.asformat();
  • clearer errors.
• LinearOperator.tosparse(format='csr') is no longer abstract either. It defaults to LinearOperator.toarray(is_sparse=True).
• Removed toarray/tosparse overrides that only raised NotImplementedError, so the defaults apply: MatrixFreeLinearOperator, InverseLinearOperator, ComposedLinearOperator.toarray, PowerLinearOperator, DistributedFFTBase, KroneckerLinearSolver.

The default costs one dot per degree of freedom, so it's meant for testing and small problems. Classes that store an explicit matrix keep their own faster versions.

Testing:

• I checked the defaults against StencilMatrix/BlockLinearOperator (including a nested block), periodic and non-periodic, all 7 sparse formats, plus matrix-free, composed, power and CG-inverse operators. Results matched in serial and with 2 and 4 MPI ranks.
• feectools linalg tests: same results as devel-tiny (the only failures are the existing petsc ones).
• Paired struphy PR (runs the struphy test suite against this branch): see the link in the comments.

See this struphy PR: struphy-hub/struphy#662

🤖 Generated with Claude Code

[spossann]

Paired struphy PR (runs the struphy test suite against this branch): struphy-hub/struphy#662

[spossann]

Added unit tests in feectools/linalg/tests/test_toarray.py (171383e). Locally: 13 passed in serial, and the 6 parallel tests passed on 2 and 4 ranks. Also bumped the version to 0.2.0 (2b30751).