Reorder fit and share denes sprs, Scafold sprs pred

Put the code for managing the constant case in fit
outside of the sparse conditional so it is shared
by both branches of execution.

Scaffold the four cases for sparse prediciton,
use a new class attribute sparse_target_input_
to carry over issparse from fit. Cases for
stratified and unifrom need improvement.
Remove temporary code from test_sparse_target_data

The code was used to test a classifier with an incomplete
 predict function.