Nlnvdg branch merge

See merge request !39

nv scheme assembly cannot take a VGF, so add some enable_if stuff to
check whether this method is available at compile time

model.py and ufl.py mostly trivial. For __init__, we make it so that
1. models can take header files directly
2. can take a different (virtual) diffusion model file path
3. model can be patched (e.g. change methods to have different arguments)
example usage in femnv module

Can now for instance create.scheme(..., parameters={"penalty":20})

cartesian storage of dofs

call the ConstLocalFunction ctor explicitly because gcc 5.4 does not

See merge request !36

Andreas Dedner authored Jan 30, 2018 and Martin Nolte committed Jan 30, 2018

Unfortunately, gcc 5 does allow explicitly constructing a tuple of
`ConstLocalFunction` from their respective grid functions, because the
constructor of `ConstLocalFunction` is marked explicit.

Note: The C++-14 standard explicitly states that this is possible.

solution,info = scheme.solve(rhs=l_h)
this adds the functional l_h to the rhs of the scheme. Note that l_h has to
be in the same discrete function space as the result of the scheme

Note: This code assumes that the ConstLocalFunction is
copy-constructible. Currently, this fails with an assertion:
shapeFunctionSet_.

on ufl expression

add `dune.ufl.Parameter`, which is a constant to be taken from the parameter file

See merge request !34

This patch allows passing definitions via
```
dune-integrands-model-compiler -Dfoo=3 -D'bar="Hello, world"' foobar.ufl
```
to the integrands model compiler.

The left hand side of the definition must be a valid Python variable
name, the right hand side is evaluated in an empty Python context.

Use the ufl error calculation instead since it is less lines of
code, and add h1 error calc.