remove `const` from ae_individual in ae_stat_record ctor

Taken from ae_stat_record.cpp:
ae_stat_record::ae_stat_record(..., ae_individual * indiv,...)

Nice. Except that the constness of indiv is abused. Later on:
ae_genetic_unit& gen_unit = **indiv->get_genetic_unit_list_std().begin();
...
_nb_bases_in_0_CDS = gen_unit.get_nb_bases_in_0_CDS()

Where get_nb_bases_in_0_CDS() is NOT const and cannot (yet) be, because
it calls compute_non_coding which alters ae_genetic_unit's state.

Hence, a bit sad, I feel allowed to remove the `const` qualifier.