Add the runtime_ufl from uflpdelab

it reserves the trial function as coefficient 0.

examples/laplace.ufl

 V = FiniteElement("CG", "triangle", 1)
-k = Coefficient(V)
-u = Coefficient(V)
+u = TrialFunction(V)
 v = TestFunction(V)
 
 forms = [inner(grad(u), grad(v))*dx]

python/dune/perftool/runtime_ufl.py

+""" This module is loaded instead of ufl when executing .ufl files.
+So, this module contains all our extensions and monkey patches to
+UFL.
+"""
+
+import ufl
+from ufl import *
+from ufl.split_functions import split
+
+class TrialFunction(ufl.Coefficient):
+    """ A coefficient that always takes the reserved index 0 """
+    def __init__(self, element, count=None):
+        if count and count is not 0:
+            raise ValueError("The trial function must be the coefficient of index 0 in uflpdelab")
+        ufl.Coefficient.__init__(self, element, count=0)
+
+class Coefficient(ufl.Coefficient):
+    """ A coefficient that honors the reserved index 0. """
+    def __init__(self, element, count=None):
+        if count and count is 0:
+            raise ValueError("The coefficient of index 0 is reserved for the trial function in uflpdelab")
+        if not count and ufl.Coefficient._globalcount is 0:
+            count = 1
+        ufl.Coefficient.__init__(self, element, count)
+
+def Coefficients(element):
+    return split(Coefficient(element))
+
+def TrialFunctions(element):
+    return split(TrialFunction(element))