added a sequential impl of forward algorithm

a few notes:

- i made the styling tweaks @calvinmm and i agreed on over gchat. i also
  changed the indentation of `hmm.hpp` to 2 spaces to match
  `data_parser.*`. should've laid this out up front, my apologies.
- i removed the `state` struct from `hmm.hpp` since really the state
  also includes `transitions` and `emissions` and just threw everything
  in the `hmm` class. @calvinmm let me know if you don't like this idea.
- i tried to use basic loops instead of iterators since they'll be
  easier to convert to openmp pragma's

ready for review by @calvinmm and @abhibhad