Accept integer function values in add_points

[Mirochill]

Fixes #46

Summary:

• normalize fx with np.asarray(...) before checking dimensionality, so Python integers and other scalar values follow the same path as floats
• update the add_points() docstring to describe array-like or scalar function values
• add a regression covering an integer objective value added to an RBF surrogate

Validation:

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