Update param documentation

*   The current wording is confusing and misleading becuase it doesn't give
    any information to the user as to why, for example, 16 is given and 64
    is returned
Refactor/Bugfix: grid sub-divide

*   cuMalloc errors raised when attempting the particle mesh step (end) of
    the sub-divide

    -   Caused by a lot of missing information and wrong variables
Merge branch 'master' into dev_mpi_cuda

*   Pick up sub-divide bug fix
refactor: send flat numpy array of grid members

*   using the `extend` method instead of `append` allows us to create a
    flat list of indexes for the nodes

*   updating the master node assignment to send the length of each
    assignment and then the actual assignment should allow us to use the
    buffering mechanism of `mpi4py` instead of using Python's pickling thus
    increasing the transfer rate (?) or time to actually transfer data