added NaN test for bessel_first_kind
added NaN tests for agrad/bessel_first_kind
added NaN and derivative doc for bessel_first_kind in manual
added NaN test for bessel_second_kind
added NaN tests for bessel_second_kind
added NaN and derivative doc for bessel_second_kind to manual
switched to no namespace version of sqrt and exp
added NaN test for agrad/fwd/erf
updated NaN test for erf
added NaN and derivative doc for erf to manual
switched to no namespace version of exp
added NaN test for exp
added NaN and derivative doc for exp in manual
added NaN test for exp2
switched to no namespace version of pow
added NaN tests for exp2
added NaN and derivative doc for exp2 in manual
switched to no namespace version of expm1 and exp
added NaN tests for expm1
added NaN and derivative doc for expm1 in manual