FREEING STUFF

   void free_xxx_params(char *params)

Always necessary if the learner uses parameters.

   void free_xxx_hypo(char *hypo)

Always necessary if the learner uses a hypo.


Additional learner fields (they may be
accessed by other functions, and they may be
set by the creator of a learner).

   bool le->uses_holdout_set

   double le->holdout_set_fraction

   paramdesc *le->paramdesc

A paramdesc is a simple datastructure that
allows a higher level of the program to know
which are the important parameters of the
learning algorithm to tune, what values they
may take while being tuned, and whether they
are relevant, given other values of current
parameters (e.g. "there's no point tuning the
momentum rate variable if the use_momentum
falg is switched off").  paramdescs come
later, so this will initially be
unimplemented.