Remove some cray pointers from `mpp/` (part 1)

[foxtran]

Description In this PR I've replaced all cray pointers in mpp_sum.inc, mpp_sum_ad.inc, mpp_transmit.inc, mpp_transmit_mpi.inc to Fortran pointers that is possible with Fortran 2008 features.

Related to #54

It is 4 of 53 commits. So, more will come once this will be merged. I do not remember everything since I rewrite it almost 2 months ago, so I'm picking only that part of work which I can keep in my fox head.

How Has This Been Tested? I have raised compiler errors by excluding -fcray-pointer flag during compilation of module. Then, in semi-automatic manner, I fixed all of them. No actual tests were running (I will let CI do it).

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] Any dependent changes have been merged and published in downstream modules
• [x] New check tests, if applicable, are included
• [ ] make distcheck passes