Remove trailing whitespace from RStan hpp files

Helps when using diff to compare code. Also, from the Google C++ Style guide:
"Never put trailing whitespace at the end of a line."
https://google-styleguide.googlecode.com/svn/trunk/cppguide.xml

To do this I just ran ``find . -type f | xargs sed -i 's/[ \t]*$//'`` in ``rstan/rstan/inst/include/rstan``

What motivates this is my frequent ``diff``ing RStan's stan_fit.hpp and PyStan's stan_fit.hpp.