Support elements with pointers in partition

[lukasdreyer]

Feature request

Currently, in t8_forest_partition, elements are put into the sendbuffer by memcpy. This does not work with elements that use pointers in their representation.

Is your feature request related to a problem? Please describe.
2.5D elements use pointers and can't be partitioned.

Describe the solution or feature you'd like
Replace copy to sendbuffer by new scheme interface function to pack elements that are next to each other (i.e. initialized in a t8_element_array, currently only there for elements created by t8_element_new).
Replace copy from recvbuffer by new scheme interface function to unpack elements that are next to each other.

Describe alternatives you've considered
The interface functions can be implemented by the default schemes with

• a memcpy (If we do not care about different endianess on different ranks)
• by providing their own MPI_DATATYPE
• by explicitely calling the needed pack functions for the datatypes

Estimated priority

"Priority: high" Should be solved as soon as possible

[holke]

This builds on the data handler works of @Davknapp and should be possible once these are implemented.

[Davknapp]

This feature currently needs a review, but we have decided to take it slow to currently focus on more urgent implementations.
Nevertheless I would love to see the pack/unpack feature in use and would support you going in that direction