Use only LocalView objects, no LocalIndexSet ones

Using LocalIndexSet objects was mandatory with dune-functions-2.5 and earlier. However, we only support 2.6 and later anyway, so let's get rid of the backward-compatibility code.

added 2 commits

• 32257796 - Add constructor from scalar
• 7b7035c0 - Use unsigned number type for VTK connectivity field

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merged

mentioned in commit e47a39b1