Allocating multiple blocks to one mpi rank in LB

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Description of changes:

• LB CPU now supports allocating multiple blocks to one mpi rank
  • The default block number per mpi rank is 1
  • The block number per mpi rank is controlled by the argument blocks_per_mpi_rank for LBFluidWalberla

[RudolfWeeber]

Pls avoid 1s in unit tests, as wrong exponents don't get cauth.

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I moved this function.

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I changed the value of agrid from 1. to 0.6.

[RudolfWeeber]

Thank you. Looks good in general.
One thing worth looking into is the cell interval business. It looks like there are a lot of very similar loops in the getters and setters for slices. Would it it be possible to pull that out into a function, which then is called with different lambdas for the individual cases?
@jngrad could you maybe take a look?

[hidekb]

Thank you. Looks good in general. One thing worth looking into is the cell interval business. It looks like there are a lot of very similar loops in the getters and setters for slices. Would it it be possible to pull that out into a function, which then is called with different lambdas for the individual cases?

I rewrote the code related to the getters and setters for slices. Similar loops are pulled into a function which calls different lambdas for the individual cases.