Change FGraph structure

[g-ferrer-sk]

After #29 , there should be a new code structure. Proposals:

1. Fgraph keeps the abstract classes plus common nodes and all factors are moved to their corresponding places, e.g., PCregistration, VO, IMU, etc.
2. all included inside FGraph

Maybe it is not necessary to include a /factors' folder, this adds some clutter.

[g-ferrer-sk]

Maybe we can remove namespaces if now FGraph will include factors from many different topics, there is no way to differentiate between them, so only mrob