cherry-pick agrad/fwd/functions/value_of from GeneralizeOperandsAndpartials Branch
added tests for value_of
updated fabs
added NaN tests for fabs
added NaN and derivative doc for fabs in manual
updated fwd/matrix tests
Merge branch 'develop' into feature/test-nan-fabs
fixed value_of NaN test
Merge branch 'develop' into feature/test-nan-fabs
fixed eigenvalues_sym, eigenvectors_sym, and singular_values test
added NaN test and doc for tan
added NaN test and doc for tgamma
Revert "added NaN test and doc for tgamma"

This reverts commit 8cf33d4076efa388e00a7a65a2b63f2f12f944a3.
Revert "added NaN test and doc for tan"

This reverts commit 451248fe2903a8621c6b729f5755c085f054a9f0.
Merge branch 'develop' into feature/test-nan-fabs
updated NaN tests
Merge pull request #965 from stan-dev/feature/test-nan-fabs

Feature/test nan fabs