Support Fluid-fluid interaction groups (#63) #61

Job	Run time
build-wasm (salva2d)	40s
build-wasm (salva3d)	53s
check-fmt	7s
all-features-native (salva2d)	5m 47s
build-native (salva2d, default)	25s
all-features-native (salva3d)	5m 57s
examples-native (examples2d, default)	7m 48s
build-native (salva2d, parallel)	45s
test-native (salva2d)	30s
examples-native (examples2d, parallel)	7m 30s
build-native (salva2d, sampling)	41s
examples-native (examples3d, default)	7m 22s
test-native (salva3d)	1m 28s
build-native (salva2d, parry)	1m 7s
examples-native (examples3d, parallel)	7m 23s
build-native (salva2d, rapier)	43s
build-native (salva2d, rapier-testbed)	5m 55s
build-native (salva3d, default)	25s
build-native (salva3d, parallel)	33s
build-native (salva3d, sampling)	49s
build-native (salva3d, parry)	45s
build-native (salva3d, rapier)	43s
build-native (salva3d, rapier-testbed)	6m 6s
	1h 4m 22s

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