Improvements in tf2 (related with documentation and style)

[ahcorde]

As discussed in this PR #258 there is some work to be done in tf2

• Rename all .h -> .hpp files. This will have to also have a deprecation warning, and should be done post-Foxy.
• Go through the comments (particularly API comments) and fix the ones that say todo.
• Go through the comments and use a common Doxygen style (some of the comments use \ and some use @).
• Fix this