Multiple blocks per process with createUniformBlockGridFromConfig

I like to use blockforest::createUniformBlockGridFromConfig to set up my block grid. Currently, that can either assign one block to each process or all blocks to one process (toggled via oneBlockPerProcess). This merge request adds a third option, selected via multipleBlocksPerProcess, that assigns multiple blocks to multiple processes. This is useful if you use the PE, which requires a minimum of three blocks per periodic direction, but they can be on the same process -- so you don't need 27 processes just because you want to have 27 blocks.

I have not done extensive testing of this yet and would appreciate comments on whether the way I set up the block grid is correct.