Return GP predictions as column vectors

[Mirochill]

Fixes #48

Summary:

• return GPRegressor.predict() results as (num_pts, 1) column vectors, matching the documented shape and predict_std()
• add shape assertions for GP predictions and standard deviations in the existing surrogate test

Validation:

• git diff --check
• git diff --cached --check
• git show --check --oneline HEAD -- pySOT/surrogate/gp.py tests/test_surrogates.py
• git show --stat --oneline --summary HEAD
• Static inspection of pySOT/surrogate/gp.py, pySOT/auxiliary_problems/ei_merit.py, and tests/test_surrogates.py
• Not run locally