[impes] Remove grid singleton from problem

Merged Kilian Weishaupt requested to merge feature/use-grid-singleton-less-sequential into master

see #431 (closed). Prepares sequential model (using old style main files) for times without a grid singleton. We pass a non-const reference to the grid to the problem. Now the Grid property is used less (it can be anywhere extracted from the GridView type) and the Grid singleton is only accessed once in the main file, so it can be easily replaced.

Edited by Timo Koch

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