Add an interaction group to specify fluid/boundary interactions #51

Job	Run time
build-wasm (salva2d)	41s
all-features-native (salva2d)	6m 6s
build-wasm (salva3d)	48s
build-native (salva2d, default)	22s
all-features-native (salva3d)	6m 14s
build-native (salva2d, parallel)	27s
check-fmt	8s
build-native (salva2d, sampling)	37s
examples-native (examples2d, default)	7m 14s
build-native (salva2d, parry)	39s
examples-native (examples2d, parallel)	7m 27s
test-native (salva2d)	24s
build-native (salva2d, rapier)	40s
examples-native (examples3d, default)	7m 29s
test-native (salva3d)	34s
build-native (salva2d, rapier-testbed)	39s
examples-native (examples3d, parallel)	7m 1s
build-native (salva3d, default)	33s
build-native (salva3d, parallel)	23s
build-native (salva3d, sampling)	38s
build-native (salva3d, parry)	36s
build-native (salva3d, rapier)	36s
build-native (salva3d, rapier-testbed)	36s
	50m 52s

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