Add an interaction group to specify fluid/boundary interactions #53

Job	Run time
check-fmt	6s
build-native (salva2d, default)	25s
all-features-native (salva2d)	5m 55s
build-native (salva2d, parallel)	28s
all-features-native (salva3d)	6m 0s
build-wasm (salva2d)	44s
build-native (salva2d, sampling)	44s
build-native (salva2d, parry)	34s
build-wasm (salva3d)	49s
build-native (salva2d, rapier)	59s
examples-native (examples2d, default)	7m 23s
build-native (salva2d, rapier-testbed)	5m 43s
test-native (salva2d)	57s
build-native (salva3d, default)	26s
examples-native (examples2d, parallel)	7m 29s
test-native (salva3d)	24s
build-native (salva3d, parallel)	41s
examples-native (examples3d, default)	7m 30s
build-native (salva3d, sampling)	48s
examples-native (examples3d, parallel)	7m 31s
build-native (salva3d, parry)	50s
build-native (salva3d, rapier)	53s
build-native (salva3d, rapier-testbed)	6m 15s
	1h 3m 34s

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