TabulatedComponent::init() may be called only once

If it's called twice, for example from two coupled subproblems, only the values are reset to zero, but not the flags that are responsible for eventually recalculating the values. Therefore, these values stay zero.

I'm not so sure what to do. We could

1. Forbid a second init call by throwing an exception.
2. Check if the parameters to init are the same as before. If yes, nothing needs to be done. If no, we could issue a warning and recalculate (by also setting the flags back to false).

Opinions?