Adds animatable class files.

hg and ccg files added.
Animatable:set_final_state(): Avoid infinite loop.

It called itself instead of the *_value() method.
Animatable: Remove superfluous returns.
Animatable: Whitespace corrections and some comments.

And I worry that the VFUNCS using ValueBase& might cause problems.
Animatable: Add interpolate_value_value() and use it in the templated method.

Also correct the constness of the Interval.