make library aware of user input for gauss_scale

input/coupling.xsd.in

src/couple_config.c

src/interpolation_method_nnn.c

src/interpolation_method_nnn.h

tests/test_field_interpolation.c

tests/test_interpolation_method.c

tests/test_interpolation_method_nnn.c

tests/test_interpolation_method_nnn_parallel.c