Tests checking implementation of checkpointing/SolverState

[NiklasVin]

These tests are written in context of Pull Request #172
Since the core functionality of the checkpointing is implemented in the SolverState class, the tests are actually testing only this class.
This commit consists of three different tests:

1. Write checkpoint & directly load it (just for the sake of completeness)
2. Write checkpoint of a function, then modify the dof vector of the dolfin function by directly accessing it via vector(). After that, load the checkpoint.
3. Write checkpoint of a function, then modify the this dolfin function via assign(). After that, load the checkpoint.

[BenjaminRodenberg]

To make sure: At the moment we expect this test to fail. As soon as #172 is merged this test should succeed. Correct?

[BenjaminRodenberg]

Some minor readability concerns. As soon as you merged the current state of develop into this PR the tests should work again.

[NiklasVin]

To make sure: At the moment we expect this test to fail. As soon as #172 is merged this test should succeed. Correct?

Yes

[BenjaminRodenberg]

Ok. Then I will merge the tests soon. This leads to failure on develop but then we have #172 as a fix already prepared.

[BenjaminRodenberg]

Looks good to me and also tests the right thing! Thanks for the contribution. Will merge this now.