MAINT: observable: move cdef extern from into pxd

Author: ev-br

mc_lib/observable.pxd

@@ -24,3 +24,10 @@ cdef class RealObservable():
 
     cpdef void add_measurement(self, double value)
 
+
+# needed to work around cython not understanding std::tuple
+cdef extern from "_observable/observable.h" namespace "mc_stats":
+    void trampoline_mrg(const ScalarObservable[double]& obs,
+                        vector[double] v_av,
+                        vector[double] v_err,
+                        vector[double] v_size)

mc_lib/observable.pyx

@@ -2,15 +2,11 @@
 
 # hack to make cythonize detect c++
 #from libcpp.vector cimport vector
-cdef extern from "_observable/observable.h" namespace "mc_stats":
-    void trampoline_mrg(const ScalarObservable[double]& obs,
-                        vector[double] v_av,
-                        vector[double] v_err,
-                        vector[double] v_size)
 
 import numpy as np
 
 from .observable cimport ScalarObservable
+from .observable cimport trampoline_mrg
 
 cdef class RealObservable():