Expose Ewald parameter to Python

Former-commit-id: dc3e794f0cb146da3ab8d8dd5e46c81997e8bc51

qpms/qpms_c.pyx

@@ -567,6 +567,23 @@ cdef class ScatteringSystem:
             else:
                 return None
 
+    property eta:
+        """Ewald parameter η"""
+        def __get__(self):
+            self.check_s()
+            if self.lattice_dimension:
+                return self.s[0].per.eta
+            else:
+                return None
+
+        def __set__(self, eta):
+            self.check_s()
+            if self.lattice_dimension:
+                self.s[0].per.eta = eta
+            else:
+                raise AttributeError("Cannot set Ewald parameter for finite system") # different exception?
+
+
     def pack_vector(self, vect, iri):
         """Converts (projects) a full excitation coefficient vector into an irrep subspace.
 

qpms/qpms_cdefs.pxd

@@ -603,6 +603,7 @@ cdef extern from "scatsystem.h":
     struct qpms_scatsys_periodic_info_t:
         cart3_t lattice_basis[3]
         double unitcell_volume
+        double eta
         #etc.
     struct qpms_scatsys_t:
         int lattice_dimension