External tasks within internal fitness provider

[marco-foscato]

The internal fitness provider needs to be coupled with an external molecular modeling task. This way we could get decent geometries to be used with descriptors calculated in the internal fitness provider AND use the expression of the fitness from within the internal fitness provider.

Also, this should be coupled the possibility of reading descriptors from the sdf file resulting from the external task. These descriptors should be defined in the GUI besides the atom-specific descriptors.