Add swri_roscpp functions for reading float values.
Add swri_roscpp functions for reading float values.

These add support for reading float values directly instead of
doubles.

elliotjo/marti_common,evenator/marti_common,swri-robotics/marti_common,swri-robotics/marti_common,elliotjo/marti_common,swri-robotics/marti_common,evenator/marti_common,elliotjo/marti_common,swri-robotics/marti_common,evenator/marti_common,evenator/marti_common,elliotjo/marti_common