Add IdentityHelper::getIdentity() #815

Job	Run time
cs-stan / Coding Standard & Static Analysis	28s
testsuite / testsuite-linux (8.2, lowest, false)	33s
testsuite / testsuite-linux (8.3, highest, false)	36s
testsuite / testsuite-linux (8.2, highest, false)	34s
testsuite / testsuite-linux (8.4, highest, false)	1m 23s
testsuite / testsuite-linux (8.5, highest, false)	1m 21s
	4m 55s

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