split Fix into XFix/YFix constraints

XFix.py

@@ -1,7 +1,7 @@
 
 from Constraint import Constraint
 
-class Fix(Constraint):
+class XFix(Constraint):
     def __init__(self, shape, pt, val):
         self.shape = shape
         self.pt = pt

YFix.py

@@ -0,0 +1,15 @@
+
+from Constraint import Constraint
+
+class YFix(Constraint):
+    def __init__(self, shape, pt, val):
+        self.shape = shape
+        self.pt = pt
+        self.val = val
+
+    def toEqu(self):
+        varrefs1 = self.shape.toEqu(self.pt)[1]
+        coeffs = []
+        for i in range(len(varrefs1)):
+            coeffs.append((varrefs1[i], (self.shape, i)))
+        return [(coeffs, -self.val)]