makes yaxt exchanger used for data exchange during interpoloation configurable

doc/src/exchanger_config.dox

doc/src/interpolation.dox

src/core/interp_weights.c

src/core/interp_weights.h

src/core/yac_core.F90

src/mci/instance.c

tests/core/test_fortran_api.F90

tests/core/test_interp_method_avg_parallel.c

tests/core/test_interp_method_callback_parallel.c

tests/core/test_interp_method_conserv_parallel.c

tests/core/test_interp_method_creep_parallel.c

tests/core/test_interp_method_file_parallel.c

tests/core/test_interp_method_hcsbb_parallel.c

tests/core/test_interp_method_ncc_parallel.c

tests/core/test_interp_method_nnn_parallel.c

tests/core/test_interp_method_parallel.c

tests/core/test_interp_method_rbf_parallel.c

tests/core/test_interp_method_spmap_parallel.c

tests/core/test_interp_weights_parallel.c

tests/core/test_interpolation_parallel1.F90