Feature/evaluate basis gradient with loopy 2

Second try to implement issue #22. We avoid problems with diagonal matrices in the following way:

1. The user can give the option 'diagonal-transformation-matrix' to the form compiler if he wants to reduce the loop.

2. It is now possible to set options during the form compilation. When a YaspGrid is created we automatically set the option 'diagonal-transformation-matrix'.

Usually it makes little sense to provide the information yourself. I just wanted to add the possiblility if someone has a axiparallel grid that cannot be a Yasp Grid (eg. due to non rectangular domain).

Closes issue #22.

See merge request !13

Cleanup/default no dot files

New default: Write no dot files.

See merge request !12

Feature/instationary

Make it possible to solve instationary problems by providing a form called 'mass' in the UFL file. Some notes:

- The most important change is in cache.py: Deleting the cache did not work. Consider the following minimal example:
```python
    global_dict = {}

    class ModDict(dict):
        def __init__(self, tags):
            dict.__init__(self)
            self.tags = tags

        def __getitem__(self, i):
            # if i == "True":
            #     return True
            return i in self.tags


    def test():
        print eval("True")

        print eval("True", {})

        print global_dict.items()
        print eval("True", global_dict)

        print eval("True", ModDict('test'))

        print eval("not True", ModDict('test'))

        print ModDict('test')["True"]

        print ModDict('test')["Blubber"]

        # print global_dict["True"] -> key error


    test()
```
The modified dictionary never throws a key error. Instead it always returns False if a key is not found in the tags. Therefore `print eval("True", ModDict('test'))` returns False!
- You can switch between implicit and explicit through formcompiler arguments.
- The time loop in the driver is hard coded. Should be replaced with something smarter in the future. -> Issue
-  In order to test the problems I compare with an exact solution. Easiest way to do that: Chose g and f like in the poisson example. Unfortunately this leads to boring vtk files since everything ist stationary ;).
- The explicit time stepping has some weird behavior. The reference solution behaves exactly the same and it is not surprising that an explicit time stepping method is not suitable for soling heat equation. Nonetheless I think it is strange how it behaves on the boundary (it doesn't try to satisfy the Dirichlet condition). The cause for that could be:
    1. I'm doing the wrong thing in PDELab.
    2. You shouldn't do explicit instationary CG in PDELab.
    3. Some bug.
- Probably it is the first option ;).


See merge request !11

This reverts commit e1bbce4c.

for style reasons

Merge branch 'feature/instationary' of http://conan2.iwr.uni-heidelberg.de/git/dominic/dune-perftool into feature/instationary

The driver solves the PDE given by the first form in forms. This is a
first step towards instationary problems.

It allows the sorting of preambles having different
cache item type.

The driver solves the PDE given by the first form in forms. This is a
first step towards instationary problems.

Feature matrix free solution of linear PDEs

This implements matrix free solution of linear PDEs. Some notes:

1. Right now the code simply uses BiCGSTAB from istl from istl for solving the linear systems. A more subtle solver setup should be postponed until the istl-ini factory arrived at PDELab.
1. Poisson DG doesn't converge. Tests with the convectiondiffusiondg lop from PDELab show that the hand written lop has the same behaviour. Because of that I turned off DG tests.
1. Right now nonlinear PDE's are not covered since PDELab doesn't seem to support nonlinear matrix free solutions. We probably have to adjust Newton first.

This merge request is related to #26 and I will update the issue accordingly.



See merge request !10

Tests/poisson variations

Test various poisson variations:

- 1D/2D/3D
- CG/DG
- triangles/quadrilaterals

This closes #19 and #18.

See merge request !9