Fix column alignment when printing Eigen matrices.

I had errors with some of the latest commits from Eigen's upstream. Indeed, printing with aligned columns was [bugged until recently](http://eigen.tuxfamily.org/bz/show_bug.cgi?id=616). As a result, roboptim-core's unit tests now fail and we have the following choices:

  1. we disable the alignment so that every user (i.e. for every Eigen version) will have the same behavior (unaligned),
  2. we keep it enabled (aligned), we fix the unit tests, but people will need to install the latest Eigen commits to have succeeding unit tests,
  3. we keep it enabled (aligned), we do not change anything, and it will be broken for everyone at the next Eigen release.

Choice 1) may break readability for people that do a lot of debugging by printing matrices in the console, choice 2) requires the roboptim upstream users to update Eigen (which is not complicated, but still...), and 3) is burying our heads in the sand.

I think that choice 1) can work (for now), unless lots of users require aligned matrices for debugging, in which case we could provide a way for users to set the IO format themselves, and force it for unit tests.