Pull request adds element properties management to class Mesh.

This is the first approach to add element properties to the class Mesh. I am not very happy that we need at least two vectors for storing values - one for unsigned value properties and one for double value properties. Until now this is a relativ easy to implement and easy to use approach. A topic for further discussion is: Do we need ```Element::value``` and ```Element::getValue()``` anymore? Another issue in this pull request is the correct visualization of the data. @bilke and @rrink will investigate the problem.