Fix the argument generated by derivative.

We previously had a false assumption that taking a Gateaux derivative
always results in adding a TestFunction. Now we add the next unused
Argument to the form, which is always correct.
ffc_interface: grab coefficients from preprocessed form

The coefficients FFC sees are of the preprocessed form, so we can't grab
them from the original form, since those may not match up.
Fix projection for dolfin-adjoint

Introduce a variable holding the solve function to be used by
project. This enables dolfin-adjoint to overload the solve call.
Merge pull request #349 from firedrakeproject/projection_solve

Fix projection for dolfin-adjoint
DOLFIN-compatible FFC parameter specification
Merge pull request #350 from firedrakeproject/ffc-parameters

DOLFIN-compatible FFC parameter specification

Allows overriding FFC parameters via the parameter system.
add UFL's six dependency to download instructions
First stab at working Python expression code
Now with passing tests
Now with documentation
propagate the value_shape name through the code
NotImplementedError for mixed functions
Grammar
Support user-defined args in python expressions

The par_loop is going to call the provided kernel with a list of
arguments, rather than the keyword arguments we're expecting.  To fix
this, in the wrapper we build around the user's eval method, convert
from the arg list into a kwarg dict.
Change the eval call to be Dolfin compatible.
Merge pull request #353 from firedrakeproject/six-dependency

add UFL's six dependency to download instructions
Merge pull request #354 from firedrakeproject/python_expressions

* python_expressions:
  Change the eval call to be Dolfin compatible.
  Support user-defined args in python expressions
  Grammar
  NotImplementedError for mixed functions
  propagate the value_shape name through the code
  Now with documentation
  Now with passing tests
  First stab at working Python expression code
Instatiation of Real function space seems to work
Include fake uid parameter
Working 0-form assembly