Enhancement: Allow multiple sensors

[jlblancoc]

Expand the template patterns to allow multiple sensor observations.

[jlblancoc]

Some old comments about how to do this:

1. Study a lot about C++ templates!! In particular, one would need
   mastering the typelist pattern design. It's very well explained in
   the book "Modern C++ Design: Generic Programming and Design Patterns
   Applied" (Andrei Alexandrescu) http://www.amazon.com/dp/0201704315/

2. In RbaEngine<>, we must replace the observation type by a typelist.
   The list only may include 1 observation, or any amount.

3. The hard part: go thru all the lib and replace anything touching
   observations, Jacobians of observations and the like, by templatized
   types. Probably the typelist pattern should be used one or more times

As you can see, it's not an "average" coding task and would require
really specialized and deep knowledge on C++ templates and
capability of abstraction. In turn, the final product will be a
metaprogramming device that generates code specifically suited to

each SLAM problem with a great flexibility.