Fix import in parse.py
Simplify magnetostatics preconditioner
Add magnetostatics preconditioner which drops Q block
Be explicit about block triangular types
Merge redundant block tri precs in magnetostatic prec
Add magnetostatics preconditioner which drops P block
Revert "Add magnetostatics preconditioner which drops Q block"

This reverts commit c30f4ff355e2869371e378656ab8d01f0f822f0e.
Revert "Simplify magnetostatics preconditioner"

This reverts commit 3b6ae515094ab7c30eada9ab803996a1f51fd28a.
Merge branch 'tests'