Release the GIL on calls to mumps

See merge request kwant/python-mumps!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 kwant/python-mumps!14

change import name to mumps and project to python-mumps

Closes #8

See merge request kwant/python-mumps!10

Closes #8

switch to meson build

Closes #11 and #19

See merge request kwant/mumpy!9