Complete solution to Schur complement

See merge request !17

Switch to pixi

See merge request !16

Release the GIL on calls to mumps

See merge request !15

Also moves some initialization operations to the `__cinit__` function for protection. Essentially `__cinit__` is guaranteed to be called even if the class is subclassed.

Marks the external c code as safe to call without the GIL

Fix handing of symmetric matrices

Closes #23

See merge request !14