supressing sign comparison warnings

eliminated sign compare warning from stan/agrad/rev/matrix/ldlt.hpp

eliminated sign compare warning from stan/agrad/fwd/matrix/dot_product.hpp

eliminated sign compare warning from stan/agrad/rev/matrix/squared_dist.hpp

eliminated sign compare warning from stan/mcmc/hmc/hamiltonians/dense_e_point.hpp and stan/optimization/bfgs.hpp

eliminated sign compare warning from stan/command/print.cpp stan/mcmc/chains.hpp

supressing sign comparision warnings

cleaning up unsigned comparison warnings

supressing sign comparison warnings

reducing more comparison warnings
adding a FIXME to add checks on inputs
removing pragmas to supress warnings
supressing uninitialized warning
using EXPECT_TRUE and EXPECT_FALSE to supress warning
removing unused variable

removing unused variable lastlp