Made append_array function work with vectors of integers (more for is…

Fixes #552. check consistent size of state and dy_dt in ode_system

operands_and_partials refactor

Need to match the appropriate forward mode derivative adjustment types
with the type present in partials_[i] even though they will never
actually be added.

Shortened test code to be more readable

Changed mixed append_array tests to be combinations of double/fvar<var> and
  double/fvar<fvar<var>> instead of fvar<var>/fvar<fvar<var>>

it was planned for use in multivariate operands and partials.