[cleanup] use std::bitset<d> consistently instead of Dune::FieldVector<bool,d>
[cleanup] rename MultiYGridSetup

There are no MultiYGrids anymore. Furthermore there are now two
versions of this function - one for equidistant, one for tensprproduct
grids. This patch gives those function meaningful names.
[doc] add documentation on mpifriendly_ygrid

The purpose of this struct has changed. This patch documents this.