replace EigenDenseBaseAddons with traits metaprogram

[bob-carpenter]

We currently use src/math/matrix/EigenDenseBaseAddons.h to define typedefs for the value type and size types of matrices. This is undesirable because it introduces an include-order dependency requiring this .h file to be included before any use of an Eigen matrix or vector.

The header src/math/matrix/Eigen.hpp then includes the dense base addons and all of Eigen. This is then included to make sure the dependency order of the dense base addons are used.

These can be redefined as traits metaprograms in meta.

• define traits metaprogram to pull out
  • size type from Eigen and std::vector types
  • value type from Eigen and std::vector types (probably base cases would make sense, too)
• replace all includes of Eigen.hpp and EigenDenseBaseAddons.h
  • replace uses of Eigen.hpp with relevant headers for Eigen in an include-what-you-use way
  • replace uses of the new typedefs with the metaprogram where necessary