- add provisional support for selecting MPI_Gatherv
when merging fields in the surface writers.
Uses the 'gatherv' keyword [experimental]
BUG: gatherv/scatterv wrappers used the incorrect data type
- incorrectly checked against UPstream_basic_dataType trait instead of
UPstream_dataType, which resulted in a count mismatch for
user-defined types (eg, vector, tensor,...).
This was not visibly active bug, since previous internal uses of
gatherv were restricted to primitive types.
COMP: make UPstream dataType traits size(...) constexpr
- allows static asserts and/or compile-time selection
of code based on size(1)
