feature class: up to 44% less memory

removed ModelDescription<2> from Feature, saving 152 bytes per Feature, i.e. 44%

Alongside, FFSimple(st) algorithms and the ModelFitter class were also removed, since these were the only ones relying on this member. 

In practical terms on a test data set with 162408 features.

Before:
Mem Usage: 126 MB
sizeof(Feature): 344

After:
Mem Usage: 94 MB
sizeof(Feature):192

Effectively saving 25% of memory (less than 44% since there is dynamic alloc of vectors which are not part of feature).

Background: I originally was looking for optimization potential in member alignment which would allow to save some memory but it seems there are no gaps which we could avoid by re-ordering members (since we rarely have char and short variables). But when looking through feature.h I found this large dead member of ModelDescription wasting 152 bytes, which was only used by deprecated algorithms (FFSimple(st)).