Add adaptive time stepping to elastic sims via enum in Variable

[ilateur]

This is a redo of pull request #57. The idea is exactly the same, but the implementation is different. It was easiest to start over.
Instead of using raw pointers pointing to parameters located in magnet, it uses enum class MaxError in Variable to remember what error to use. The actual values of the 3 errors are now stored in the TimeSolver class, along with all other adaptive time stepping parameters. I think/hope this solution is cleaner.

Still need to take a look at sensibleTimeStep().

[ilateur]

Some todo's I should not forget:

• take a look at sensibleTimeStep()
• add a test comparing results with a very small step size
• may need to tweak default values 1e-18 and 1e-7 of displacementMaxError and velocityMaxError