Add nogil declaration to _run_paradiso function

This releases the Python GIL around the low level paradiso solver call.
Hopefully this allows this library and dependent libraries (like SimPEG)
to operate more smoothly in a multi-threaded environment.

This should be safe from a Python object perspective because no Python
objects are created/destroyed/inc-reffed during this call.  It could
still be that multiple concurrent calls of this function on the same
data cause undesried results, but hopefully downstream users can apply
concurrency primitives (like locks) as needed.

Author: mrocklin

pydiso/mkl_solver.pyx

@@ -530,7 +530,7 @@ cdef class MKLPardisoSolver:
         if err!=0:
             raise PardisoError("Solve step error, "+_err_messages[err])
 
-    cdef int _run_pardiso(self, int_t phase, void* b=NULL, void* x=NULL, int_t nrhs=0):
+    cdef int _run_pardiso(self, int_t phase, void* b=NULL, void* x=NULL, int_t nrhs=0) nogil:
         cdef int_t error=0
         cdef long_t error64=0, phase64=phase, nrhs64=nrhs