Feature/arbitrary virtual refinement

This merge adds more flexibility to the VirtualRefinement class. The old version of VirtualRefinement could only produce 2^n sub elements per axis. For example, refined triangle with n=2:

|\
| `\
|   `\
|-----`\
|`\   | `\
|  `\ |   `\
|----`\-----`\
|`\   |`\   | `\
|  `\ |  `\ |   `\
|----`\----`\-----`\
|`\   |`\   |`\   | `\
|  `\ |  `\ |  `\ |   `\
|----`\----`\----`\-----`\

Now it is possible to have an arbitrary number of sub elements per axis. For example, a refined triangle with 3 sub triangles per axis is possible:

|\
| `\
|   `\
|     `\
|-------`\
|`\     | `\
|  `\   |   `\
|    `\ |     `\
|------`\-------`\
|`\     |`\     | `\
|  `\   |  `\   |   `\
|    `\ |    `\ |     `\
|------`\------`\-------`\

To use the number of sub elements in a call of a VirtualRefinement function an RefinementIntervals object must be passed, instead of just one int. For example, to call nVertices use

nVertices(Dune::refinementIntervals(n)).

To use the old functionality, call the same function with Dune::refinementLevels(int), which creates an RefinementIntervals object with 2^n intervals.

Calling the VirtualRefinement functions with one int yields the same result as before, but will additionally give a deprecation warning.

The changes above are now also implemented for the StaticRefinement interface.

Intended merge date: 2017-08-28 (@joe).

Closes: !56 (merged).