Avoid round-off errors on U-matrix diagonal.

Diagonal elements of Lattice.isotropicunit should be exactly 1.
Matrix multiplication could introduce slight round-off errors.
This had caused inexact assignments of U_ii for isotropic matrices.

Author: pavoljuhas

diffpy/Structure/lattice.py

@@ -197,6 +197,10 @@ def setLatPar(self, a=None, b=None, c=None,
         self.normbase = self.base * [[ar], [br], [cr]]
         self.recnormbase = self.recbase / [ar, br, cr]
         self.isotropicunit = numpy.dot(self.recnormbase.T, self.recnormbase)
+        # ensure there are no round-off deviations on the diagonal
+        self.isotropicunit[0, 0] = 1
+        self.isotropicunit[1, 1] = 1
+        self.isotropicunit[2, 2] = 1
         return