added fvar_inner_type and partials_return_type

Used to determine return types for prob functions using fvar
renamed fvar_inner_type to partials_type and added partial_return_type
added first version of OperandsAndPartials for fvar
fixed partials_return_type to give scalar values
fixed traits tests
added to_fvar for OperandsAndPartials with test
normal distr breaks with new return statements
added generalized OperandsAndPartials
removed agrad/fwd/prob/.../normal
fixed normal.hpp so normal_log works with fvar
added more tests for normal distr with fvar
generalize DoubleVectorView
generalize internal_math for prob functions
fixed other distributions for new OperandsAndPartials
added is_fvar_var and is_fvar_fvar_var with tests
removed extra template parameters in distribution testing
added fvar signatures to unit-distr/utility.hpp
fixed partials_var typo and added more normal tests
generate_tests now generates all tests for rev and fwd mode distributions
added is_fvar_double and is_fvar_fvar_double to traits