Use FeatureSpaces as FeatureValue factories

[b-adkins]

Why?

• Check FeatureValues validity at initialization. Thus, invalid FeatureValues DON'T EXIST. (Perhaps a pointer in each FeatureValue linking to its FeatureSpace? This could compile out for speed, but would be useful for reliability.)
• FeatureSpaces know how to create valid FeatureValues for themselves.
• Possiblity: specialized ROS callbacks to create FeatureValues, which are passed to polymorphic code that doesn't anything about them.

Changes:

• Add the following methods to appropriate FeatureSpaces:
  shared_ptr FeatureSpace::createValue(int discrete)
  shared_ptr FeatureSpace::createValue(std::vector belief)
  shared_ptr FeatureSpace::createValue(std::vector vector)
• Make FeatureValue constructors private.