adds support for configurable yaxt exchanger to user interface

doc/src/reference.dox

python/yac.pyx

src/mci/instance.c

src/mci/instance.h

src/mci/yac.c

src/mci/yac.h

src/mci/yac_global.F90

src/mci/yac_module.F90

tests/mci/test_dummy_coupling9_c.c

tests/mci/test_dynamic_config.F90

tests/mci/test_dynamic_config_c.c

tests/mci/test_instance_parallel2.c

tests/mci/test_instance_parallel3.c