Add explicit feature generation tests

tests/.~lock.data.csv#

tests/test_feat_generation/test_abs_diff_node.py

tests/test_feat_generation/test_abs_node.py

tests/test_feat_generation/test_add_node.py

tests/test_feat_generation/test_cb_node.py

tests/test_feat_generation/test_cbrt_node.py

tests/test_feat_generation/test_cos_node.py

tests/test_feat_generation/test_div_node.py

tests/test_feat_generation/test_exp_node.py

tests/test_feat_generation/test_inv_node.py

tests/test_feat_generation/test_log_node.py

tests/test_feat_generation/test_mult_node.py

tests/test_feat_generation/test_neg_exp_node.py

tests/test_feat_generation/test_sin_node.py

tests/test_feat_generation/test_six_pow_node.py

tests/test_feat_generation/test_sq_node.py

tests/test_feat_generation/test_sqrt_node.py

tests/test_feat_generation/test_sub_node.py

tests/test_sisso.py